318 or 318 ?? which one

IIRC, the '68-70 318 was rated at 230hp. And the '74 was about 125hp(?).. Not an inconsequential number I drove (long term) both a '70 Swinger and a '75 Duster. The performance differences were much more than "minor".

A previous poster suggeted that the '74 engine would have hardened exhaust seats. If this is more than just an occassional driver, personally, I would consider swapping on the later heads. At the same time, consider swapping in a set of aftermarket roller lifters and comparable cam, along with a decent 4bbl intake and carb. For a relatively small investment, you should see some real improvements, possibly even exceeding 300hp.

My .02 cents.
Mike in FL