Im putting a big block in my 68 and was wondering if someone could tell me which water pump to use long one or short one

 

Your car came with the short pump, and alternator on the drivers side. Chevy put the long pump in the Camaro starting in 69, with the alternator on the passenger side instead. Unless you want to redo the wires for your charging system, it would make more sense to keep using a short water pump.