Depending on how much the ports mismatch,there is power to be gained with a port match. It made a HUGE difference on a '75 Toyota I had. They were off by 1/16". Never dynoed it but it never lost speed on a hill in 5th gear again. Best part - it only costs time unless you don't already have a die grinder. If you plan to use a 2 1/2" exhaust open up the manifolds if there's room.
Ron