I’m running the stock 340 exhaust manifolds. 410 CI at 6400-6500 shift, stripe at 6800. Will it make more power with headers? Sure, but it would be WAY down the priority list to getting the highest flowing head you can. It a sorted car, you can pretty easily run mid 10’s with those exhaust manifolds. Molnar,K1,Scat forged rotating assembly. The highest flowing aluminum head you can get. Go through the 904 so it will live(not that hard to do) Solid flat tappet should get you where you want to be. I recommend an EDM lifter. Talk to a good cam company and get that settled. Then talk to converter company, I just got a 9.5” PTC for my 904 with a pretty conservative flash rpm. Very happy with it, the lower flash helps with my little tire, but would also help a street car that lives on slick roads.