HI,
I put a Banks stainless steel exhaust system on my nearly new 1997 Bounder motorhome with a 460" V8 Ford chassis, just because Banks said it would make more power, and the factory manifolds tend to crack and leak. I did not want a 1,000F exhaust leak near the wood frame that is close to the engine!
So far my stainless steel exhaust has held up well, no leaks in about 50,000 miles. I gave the old system to my mechanic, who had a 65 Econoline pickup with a 460" engine in it.
I would highly recommend the Banks system, even though it is probably more expensive than a equally good working header by another manufacture, such as Gibson or perhaps Edelbrock?
Good luck on your project. I was going to recommend looking for a vacuum leak, but I guess rebuilding the engine would have solved any prior intake manifold leaks?
You might also check out the distributor. I had one rebuilt for my 75 F-350 and it ran like a champ once rebuilt. Clutches Unlimited in Midway City, CA did the work.
Fred.