I think you've gotten good info so far, though I disagree with a couple points.
I used to have a 63 Biscayne 4 door.
It had a 250 swapped into it.
With a PG trans it got 21 mpg on the highway.
As far as them weighing 4000 lbs.
see:
http://www.carnut.com/specs/gen/chv63.html Early 60's Chevys all run to the light side, but only got heavier with each passing year it seems.
If this is the original engine consider pulling it and tranny out to save and swap in the warmed over 250 or 292 and 4 speed auto.
I think that rear should be a 3.08, so maybe the gear swap is a good idea. If I went with a mostly stock 292 I'd leave the 3.08, I think it would pull just fine.
see:
http://www.tciauto.com/tech_info/gear_ratios.htm The TH-350 is tempting as it's not hard to find one cheap,and they almost fall in place. It helps launch the car better than the PG, and you still have a passing gear at 60 which is a novelty if you been driving a car with a Powerglide for a while.
But you still get to watch the rpms go up, up, up while whizzing along with the Atlanta traffic. That's how it worked here in SE Penn.!
If you go the T-350 route, don't use the big stock convertor! Whatever trans you use check that center bearing so you don't have driveshaft problems.
