I used the flat top manifolds on my '56 Fairlane 312 +.40.
I went to a local muffler shop and they did the whole system front to back.
I think it was $250.00, several years ago. I also used weld in glass packs. I don't know the brand but they sound great.
The cam break in can be very bad. I lost a new Isky cam in 20 minutes. I know this sucks, broken lifters which required a complete tear down and cleaning. BE SURE TO USE OIL with the 40 weight rating. This oil will have the greater amount of PTFE and ZINC. We have threads on this sight with the information. Just do a search on ZINC in the oil.
If it was mine I would disassemble and re-lube the cam and lifters. But that's just me.
Good luck! You will love the way it runs. Expect to go thru some tires. ( Burn-outs)
Jim
Jim Rowe Elkhorn, NE
