cool project, you will find there are alot of parts available for them. Yes I am on the 67-72 website under the same user name, stop in and say hi! There are thousands of members almost like a cult These trucks can be made to go fast.
Yes on the retune, I did some logging after the first tune and am working on the second retune after the swap. I am using exhaust manifolds from a Z06 they look like shortys, before the swap I read an article in CarCraft that did some comparison and found (in the article) that they compared favorable on a dyno with LT headers. Now that being said there may be some or quite a bit left on the table with them in the "real world". I have been running them since the initial swap.
Unfortunatly headers are pricy and I am not sure what if any would fit. I installed the motor rearward as much as possible using custom fabbed mounts. I dont know if I have room? These dump into 2.5 inch duels.
Dave