I have a short on my 62' but I think there would be enough room for a long pump. If I did it again I would probably do a long because the acessories like the alternator sit in closer to the engine for a better fit. I still like the short because it is what they would have used if a V8 was an option in the early ones.
If I had it to do over again, I'd think twice before putting a long pump in an early Chevy II. No one makes a "proper" alternator mount, unless you like the 69+ OEM type, and fine tuning the radiator & fan setup is a huge pain in the butt.
IMHO, You'll have more/better options with the short pump.