340 Exhaust Manifolds on Early A-Body??

If you have not done perfromance work to 273 flowing more with exhaust may have negative results for performance. If the car is original why mess with it? If not I would put in a 318 and make it go better as I have in my 65 formula s, I also did that in a 66 cuda that with some work ran 14.0 at track. stock lower end , mild cam, new lifters and pocket ported heads with stock 318 exahust (dual)...put out about 300-320 hp and was lots of fun. Blew up small rear and custom made new 8 3/4 for strip was then going to make motor go more but an old man ran into it. I wish I kept the rear to put in my current 65....