You wouldn’t see much of a difference.I’d change the plugs maybe once a year or if they’re fouled,always used Autolites and my 355 ran good.Wires should be still good.
Removal of pollution control parts
Exhaust/Headers
Intake Manifold
Better Carb
Gears
Cam
Thats what I’d do,probably in that order.I dont think the k&n is worth the price unless you can get it cheap.