Port your 273 manifolds, I have used Muriatic acid to get deep inside areas and some grinder work no real cost as apposed to hunting down 340 manifold $$$ for minimal gains. I understand wanting PS who wants to drive dump truck thru a parking lot, I can rip a phone book in half (well almost).
I think the problem lies in the fit, where the 340 heads have bigger ports than the 273 manifolds and exhaust was leaking by. Are there any gaskets that can solve this problem?
