I'm sort of in the same boat as you with similar mods. Stock 340 with x-heads and 3.23's. Performer RPM manifold. My manifolds are not the hipo ones though but rather 72-73 dated manifolds with the exhaust coming off the passenger side looking like less than 2". I am for sure doing headers this Winter, just haven't decided yet between TTI's or Doug's. I think you should go with the stepped TTI's.
