340 manifolds, the '69-'70 models, are going to flow a lot more than stock 318 manifolds. But they are not cheap. For around the same money you can get a set of Dougs headers. But if you go with 340 manifolds and want to upgrade to headers later you most likely get your money back out of them. The other thing about 340 manifolds is you will need different head pipes where as if you with Dougs you would be able to hook your current system to the headers without much trouble.
