Early A-body exhaust

Huh? Find a shop to design the core boxes and cores, and produce those cores and boxes for casting manifolds will cost you about $10, 000 per side. Then find a foundry to cast them and a machine shop to machine the surfaces and then we'll talk. Someone already reproduces the 340 manifolds and you can find used ones at swap meets although they aren't cheap nowadays. THere are also threads here telling about a combination of stock manifolds that work and fit pretty well. TTI makes head pipes for a list of manifolds you can use that hook to their dual exhaust set ups. There are a few headers available. Hookers, Dougs, and even Layson's makes a set that are shorties. They may be one step better than the 340 manifolds but I haven't seen any dyno tests on them. No reason to reinvent the wheel unless you are made of money.