After buying a malibu that was supposed to have a 350 in it. I have access to two 350's take my pick.
The one I know for a fact is from a 78 chevy 3/4 ton van 4brl, casting number 3970010, or
a motor that's a bus motor 2brl, casting number 14016379. I understand any 350 is better then a 305, for performance.  

There isn't any significant differences between the two blocks. Use whatever has less wear and tear on it, or if don't have any idea flip a coin.