The strong vacuum in the crankcase is odd but I don't know I'd focus on that first.  If the engine and truck is unknown, I'd start with clean fuel and filters as well as rebuilding/cleaning the carb after sitting for 15 years if that hasn't been done already.  Then pull the plugs to inspect for oil fouling, wet from gas, gap, etc. and while out, do a compression check for a baseline, and hook a vacuum gauge to and see what that tells you.  Sorry if these were the general things you'd already mentioned were done.

You mention the vacuum lines.  Does it still have what I assume is a complicated spaghetti from the stock 1986 setup?  I'd definitely be taking a lot of pictures and notes before touching that.

Pat

Make sure the choke is all the way open and fast idle screw isn't touching the fast idle cam.  I had that happen on mine.  The choke wasn't open 100% but I thought it was open enough the fast idle screw would not be in play but I was wrong.

Next thought is that the advance springs in the distributor are sometimes not pulling the mechanical advance out when you are at idle.  More timing equals faster idle.

Finally, next time it is idling "fast", go under the hood and make sure the secondary plates are all the way closed.  Primaries too for that matter.  See if you can rotate them any more shut, indicating they are hanging slightly in the bores.

Pat

Looks great so far.  I'm waiting on the prop for the front to complete the look.    At the speeds I anticipate you'll be running, you wouldn't even need to power it - air resistance would spin it.

Pat

I'm not familiar with the Packard transmission, but a friend of mine had a VW Beetle that worked similarly.  You had to shift it (I think it was a 4 speed but might have been a 3 speed) but there was no clutch pedal which made me nervous the one time I drove it.  You just accelerated in first gear, then backed off the throttle, shifted to second, then got on the throttle again.  Kind of interesting...

Pat

A couple of years ago I worked part time at a Chrysler/Jeep dealership as a parts driver.  But there was a lot of time when there was nothing to deliver or pick up so for those times I worked the service line as a "valet", meaning I had to pull the customer cars out back to either the main service lot or the line for the tires/battery/radiator/lube shop.  Months would go by between when I'd see a stick (but again, only working part time and then only when not doing my regular delivery work).  But when we did get one, no one seemed to have a problem with it, which kind of surprised me but in a good way.  The guys in the lube shop pulled them on the lifts without remark and so did the technicians in the regular shop.

Pat

Wow, that's impressive!  To the rest of us that only see that maybe once or twice in our lives, it looks like you are at ground zero for some alien invasion like War of the Worlds.

Pat

So I went to napaonline.com and allowed Firefox to share my location, so their website could detect my nearest store which it did, about a mile or two away.  I then clicked on "add a vehicle" and put in a 1988 Ford Mustang.  Then I went to the search bar and typed in clutch.  One of the automatic entries that came up below the bar as I was typing was "clutch kit".  I clicked on that and got a number of clutch/pressure plate/throwout/pilot results, followed by A/C clutches, and other non-matching results.

Now, some of the clutch kits were out of stock, some were in stock and some could only be shipped directly to me.  All of them had a blue warning next to them that said they could not guarantee fitment without additional info.  I clicked on the blue warning on one of them, and it popped up asking me for the engine.  Two choices, the 6 or 8.  I picked the 5.0 V8.  It then popped up asking the transmission.

Sure enough, there were two 4 speed automatics listed and no manuals.  BUT at the bottom of the list were "I don't know", "Not listed", and maybe another one that I forget.  I chose "Not listed".  The popup closed and I was back at the search results page showing the clutch kits and now all of them had green check marks by them and said exact fitment guaranteed where the blue warning was before.

So I did think it was odd that a manual transmission wasn't called out as an option though.  It was like they were saying that as long as you don't have an automatic transmission, then these clutches will work.  Kind of reverse logic. 

Pat