Go with a roller cam late model 302. You can put a small four barrel and a Performer intake on it with an aod trans and get 25+ mpg on the freeway with 3.55 gears.

But that's my opinion because I got 27+ mpg at eighty mph with that combo in a 64 Fairlane.

Sure the 221 or 260 sounds like a good deal but they are in short supply lately. Any that are rebuildable are wrapped in plastic in garages in case their owner decides to pull the roller 302 and do a full restoration.