I would also vote for keeping the original engine / transmission combination. Depending on where you live, a complete engine rebuild (including balancing and blueprinting) is not any more than a new crate motor, in my area (Reno, NV) easily less. Transmission rebuild (if needed) is entirely different. Depending on what tranny you have, expertise to properly work on a 38 yr old tranny is often, in my experience, harder to come by than a competent engine rebuilder.
In any event, good luck and enjoy your ride!