I debated the rebuild/replace dilemma for a while, but opted for the 350. Granted it isn't in yet, I figure my LT1 should get 20,... possibly + miles/gal, whereas my F got 4-7 mpg on the freeway, was floored most of the time (lots of hills pulled @ 40-55 mph pedal to the metal) definitely 7 mpg on level ground at anything above 40-45. If you're familiar with WA, it takes me a FULL tank of gas to get from Yakima to North Bend ~100 miles.
Towards the end of the summer, when I could see the smoke of my engine's blow-by in the headlights of passing vehicles pulling some hills on the freeway near my home, I decided it was time for something new.
If you're not in a rush, you could buy several Chev 350's for what it costs to rebuild an F, or a 2F. I got my LT1, pulled from a '95 police cruiser w/100k mi for $350 (I sold the tranny to a buddy for $100), so I got the engine complete with harness for $250.
My buddy poohbair has had several running 350's given to him, for the price of getting the nasty, beat-up trucks out of their owners' yard,... he just pulls the parts he wants, and has the local wrecking yard remove the rest for $35. The Chevs will also give you 1k more useable rpms at least, which definitely come in handy.
Cruisers (F & 2F alike) were designed to run at 50mph literally forever,... if you want any more speed you've gotta rethink it.