You are in an interesting middle point of a nice engine that may not be as fast as you want, but could run really well
The 270H, RPM, stock head combo will run like a dream. It's a nice street combo with a little chop, behaves nice, and if well tuned will be fun.....I have personally ran that combo hard and it's a sweetheart to take out on the road. I would consider a distributor recurve, likely 12-14 initial 38-40 total, all in by 2800-2900, it really wakes up part throttle but doesn't do much for racing
However, the parts probably won't get you in the 13s without perfect suspension and a little luck, the list from worst to best
1 - Heads are a maybe - If C6AE-R, they are OK stock, but not great, if C6AE-U, they are 220 cfm heads and hurt you significantly, both in overall power and cam requirements. Make sure they don't have two piece retainers, those are so heavy, unlikely if GT heads, but it's been a long time since 1966
2 - Manifolds - Probably a 35 hp loss over headers and drives even more change in the cam
3 - Cam - If you had good iron heads and got the backside breathing, it could use some exhaust duration, but IMHO, not worth doing if the other two aren't corrected, and likely would meet your goal if the other two were corrected, just more in it You can cam it up, but if bad heads it's tough.
Fast guys run good heads...
BREAK BREAK....I just saw your new plan...very good, but depending on your current heads, the FPA 16 bolt may not work, look close at gasket sealing. If C6AE-R you should be OK, C6AE-U make leak at the bottom of the port due to low exhaust port location on the head and poor gasket crush
Realize that as you go more modern and capable with heads and cam, you may find piston to valve clearance too. So your plan to run it now, regroup and plan a new engine is a very good idea
Sort of like the old "Stone soup" fable
