What actually determines when to use the long water pump rather than the short one? I have a 69 camaro, 350, p/s, a/c and assume I need a long since I don't have my pulleys or brackets yet.

If you're going by how they built it, the model year determines that. All Chevy's 68 and prior (and vette's 82 and prior) had drivers side alternators, short water pumps, and heads without accessory bolt holes in the front.

All 69 and newer had long water pump, pass side alternator, and accessory holes in the front of the heads.