Personally I would pick any good condition 318, 340, 360 or 383 that you come across for a good price...that can be rebuilt.
Going from a /6 to V8 you need a bunch of stuff anyways for the conversion so to me big block or small block is kind a mute point.
Blowing the budget...
Looking at Big Block VS Small block weights...seems like you could level the playing field by putting aluminum heads on a 383 and that would drop 30 - 60 pounds? making it closer to the weight of a small block?
All I can say is...it's your car...go with your heart and gut feeling...and what makes you happy.
