So, first, for run-on
Run-on after shut down is due to too high of an idle combined with an oxygen source. This is typically the throttle blades are too far open at idle, because some other part of the build drives it.
Some thoughts and reasoning
1 - Usually, the primaries are too far open, but it can be the secondaries on a 4 corner idle too. This is really "dieseling" because the engine is firing on residual heat, combined with fuel and air being pulled in because of the carb being misadjusted. But...how do you get it to idle down?
2 - Initial timing - The higher the initial timing, generally the higher the vacuum and the lower the engine will happily idle on its own. If you are 100% positive of your TDC, then just make sure with a cam like that, that you have somewhere north of 14, and if more, it's likely easier, but you don't usually NEED more. Just to make sure you have the TDC checking correct, the only exact way now is to check with a piston stop. You put a piece of tape on your balancer, all the way around, masking tape is fine. Thread in a piston stop, turn the motor backwards by hand off the crank until it hits and stops. Mark the tape, turn it the other way hut it hits again, mark the tape. Measure the 1/2 way point between the marks, that is true TDC and should match your balancer. If it doesn't, either the balancer is bad (if its way off) or could just be manufacturing differences or parts choices, just remark it
Also, timing is set and forget, unlike some steps below. Pick a curve, set it up and rule it out. I would go 16 initial after you verify TDC, likely a 16-18 bushing, and then choose springs for all in by 2700-2800. Verify, then leave that part alone
3 - Adjust your carb correctly
You will likely have to go through these steps a couple times, and in this order. Once you go through, if it idles too low or high, you have to go through all the steps again because A affects all
A - Usually, you take off the carb and look at the blades. The transition slots should just be showing with the carb NOT of fast idle. Two goals here, front and back. Transition slots barely showing (allows some vacuum to "preload" the circuit for when airflow kicks in, but not enough to pull fuel and not enough to let extra air in. You adjust by the throttle stop screw on the rear, and on the front the idle speed screw. Sometimes you have to do this a couple times because in the end, you would rather have the rears slightly open because the transition slot is a little higher and its not as fussy. So bottom line on this, close up all 4 throttle plates to where they should be, and if required, bring the idle up slight later using the rear
B - Adjust idle air fuel correctly, start at 3/4 turns out if 4 corner idle. Start the engine and watch manifold vacuum or listen with your ear. Go in and out and watch what each screw does. You want highest idle or highest vacuum. Go in and out and repeat as many times to be sure. I use my ear, but new guys often like a vacuum gauge. However, be sure that all screws end up the same. You should not be more than a 1/8-1/4 off that 3/4 initial setting, and usually you can almost just guess it there and leave it to be honest. Same if it is 2 corner, but start at 1 1/8. If after setting the timing, throttle plates and a/f, you cannot get it to idle, take the carb off and raise the rear idle a bit, very little, then go back and start over with the idle a/f at 3/4 or 1 1/8 depending on your carb.
C - Make sure heat is managed properly - A carb can lose it's ability to control fuel if boiling over. Although this is generally not a cause of diseling, it can do some odd stuff if it gets hot. So if you cannot easily tune it and it feels real hot at the carb, if you do not run a spacer, add a 1 inch phenolic, then start over with carb adjustment. Keep in mind a 67-72 truck with an RPM intake doesn't have much room at the cab lip, so you may find you need to dent the air cleaner element or get creative if you put a big spacer with an RPM intake. BTs and SD/SM do not have that issue
Next, the cam. Will it help run-on?
Sort of, and yes, but not before what I typed above.
So, if you increase idle vacuum by any means, you can close the carb more and idle the engine down a bit more. So, advancing the cam will do that, assuming everything is correct above. However everything above needs to be done or it can still diesel. So, if you decide to advance the cam, then you have to recheck timing and readjust the carb. The two major factors are carb and timing, so it sort of drives you to either try it, or just advance the cam now for a little better manners and then do your distributor swap, then your carb. Again, it's not a magic bullet, but I would likely advance that cam for better street behavior. Knowing perfectly well you proved that it wasn't your "problem" but at this point, it's apart and you can make it a little better, and in some conditions, noticeably better
Other questions
1 - The Ford racing timing set will let you advance in 2 degree increments. I would go 4 degrees advanced slot, and then degree it for open/close. It should be 13/41, or as close as you can get. Srould you go more if it ends up at 12? Depends on the valve clearance checck
2 - How do you check clearance? I put checking springs on the #1 cylinder's valves, although you can get away with just the intake valve on yours because the cam has already ran with the exhaust and advancing only tightens the intake, it'll make the exhaust better You can use a Ford rocker shaft spring. You do not need or want to assemble the rockers, but you do need to have the balancer on or the degree wheel set up for true TDC to reference TDC
Once its assembled with the soft spring, bring it to 20 degrees BTDC. Measure installed height for the intake valve with a closed valve (retainer to bottom of spring) then push down on the valve tip (not the retainer it will pop off) until it hits the piston. Subtract the numbers, that is how much total room you have. Subtract valve lift from that and you get clearance, write that down. Some might say to install the rockers, that's good with a solid cam, but for a hyd cam just measure
Do the same procedure at 15 BTDC, 10 BTDC, 5 BTDC, zero, clearance will shrink, repeat until it starts growing. You now have a map of your clearance, it will likely be tightest at 10 BTDC or so.. It took as long to type as it does to check it. Measure, turn, write down, repeat, then go do the math and if one set of numbers doesn't make sense, go back to the truck, and recheck.
As long as you are over .090 on the intake side, you will be good, and you can go tighter, but I doubt you are even close to that. FYI, to repeat, the exhaust side it is more critical, but remember, it has not blown up yet, and advancing the cam gives you MORE room on the exhaust, so unless you want to, I wouldn't even look at it for this exercise
Once you advance that cam, set the timing (with the new distributor) and adjust the carb, life should be real good.
Question - I have been struggling to think about why the timing jumps around. Could be a module, could be vacuum advance with the plates too far open, could be a bad rotor, distributor cap, if you have a rev limiter, I have seen pills go bad, or #1 spark plug wire is failing. All of these would show on a timing light. Have you checked any of those other things? I have never seen a module bounce spark around unless it had a built in limiter that went bad/
