A pair of magnum heads could wake it up quite a bit in the air flow department and compression and the 1.6 ratio rockers will make the cam work like it is slightly bigger. Make sure you use the thin head gaskets and you WILL notice a differance. You can get stock re-built heads from flea-bay for a couple hundred bucks, have them re-drilled at a local machine shop for under $100 (If you were close by I only charge $50), buy a gasket set, conversion pushrods from mancini and be done, then you can keep you stock short block in the car and it will make more power than a 68 340.