a 350/350 combo is a winner for sure, parts are super cheap and they're easy as hell to work on. My '78 chevy had the same drivetrain and it was bullet proof. I want another one...whatever you do, don't buy a Haynes manual. They SUCK! Get the Chiltons.
I just got a 2001 Dodge 2500 w/ the Cummins diesel and a 5sp manual back in Feb, only 70,000 miles on the clock, so I sold the '78 cause I also have a 2006 VW Jetta TDI (diesel). I'm a diesel-dude! Trying to sell my car though...need more bike money!
Love them old trucks though, similar to our bonnies really, classic, fun to work on, sound great, look great, feels cool cruisin in them. I want another one! Only prob with those big V8s, like you said, is the MPGs. My truck got like 8mpg no matter what! It had a 4:10 rear end though. My Dodge gets 18-20 mpg, but it's a diesel.