TTi step vs. 340 manifolds
When the heads are in the shop have them mill them as much as possible if you still have the stock 73 pistons. Have them mill the intake side also. This alone will be your biggest bang for the buck. Also seeing you may have the small manifolds if you have the $$ for the headers I go for them. But again milling the heads will make it feel like a different motor all by it self.