The left side was the problem when I did it too, nice that you have the body off the frame, makes things easier for this operation.
The class I raced in required cast iron manifolds, no headers allowed, and I had previously had a 70 Judge RA III I used the manifolds off of that car. This was long before RARE was in existance, it was 1977. Finding a pair of 67 RA manifolds was at that time like looking for hens teeth, hence the reason for adapting the 70 manifolds to the 67 chassis.
Well I've spoken to Paul at Pontiac events, and he is probably fairly knowledgeable about his own products. I would have thought that Pontiac would have designed the manifolds very similar between the III and IV engines, but maybe not.
If you decide to setback the engine it's only going to be accomplished by removing the mounts from the frame, moving them rearward, and drilling, or slotting the holes in the crossmember. There is no combination of the engines soft mounts that is going to get you a setback. This usually requires removing the lower A frames from the chassis to access the nuts on the inside of the crossmember. There are a few cars that the nuts can be accessed without removing the lower A frames, but it still isn't easy to remove and replace them even if that chassis affords you access.
You might want to look at a stock 64-67 A body to see how much firewall clearance you have. Just gong from memory the 67 has adequate clearance. First gen F bodys have really tight clearance in that area, but A bodies have more clearance.
As engine masters has already proven on the dyno, denting a header primary tube for clearance has little if any effect on power. I've done it many times over the years, mostly on Pontiacs, but other brands as well.
Good luck in whatever you decide...........
