If it was me, I'd go with a 350. I agree with MN51 in that an HEI (or Pertronix Ignitor) really DOES make life easier. And a PCV valve is a good idea. Much cleaner than a road draft tube.
I also agree about using a carb with small primary venturis (Edelbrock, Carter or even Quadrajet.) If you can keep yer foot outta the carb, you'll get pretty much the same mileage as you would if you were running a 2-barrel carb and still have some power on tap if you need it.
As a general rule, when people consider swapping a V8 into something (car, truck, hot rod...whatever) economy generally isn't the primary concern. Performance is. That's probably why you haven't been able to find much on it. But economy comes from a lot more than the way the motor's configured. As MM51 sez, tall gearing like 3.08 or an overdrive transmission coupled with a light foot on the throttle will provide the best possible mileage (as another general rule) with any motor combination.
As far as which motor you use (305, 327 or 350) you'll probably get about the same mileage. I suggested the 350 because they're more common and easily obtainable than 327s and have a much greater performance potential than a 305. I know a lotta guys who put 305s into street machines and they're ok with them if they just want a V8 that is more of a cruiser or transportation vehicle...but if they want to increase performance, most of them will go with a 350 (or 400 smallblock) eventually.
If you really wanna go small, you could try to find a 267 V8 from a late 70s/early 80s GM product and run it...but I don't believe that it would increase mileage all that much...but it sure would limit performance. Like real slow.
Those are just some ideas off the top of my head. Good luck with whatever you choose.
