I'd suggest a 292 engine, Turbo 700 overdrive automatic transmission, and an open-driveline rear axle somewhere in the 3.50:1 range. That running gear will get you into the mid-70's to mid-80's date range, and the 292 can be built into a real powerhouse if you choose. You'll need to fabricate some motor mounts, and possibly change radiators, but the upgrade isn't nearly as difficult as a swap to either V-6 or V-8 power.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!