So what the difference (besides the obvious, of course) between a long pump and a short pump?

The complete pulley and belt configuration.You have different brackets(mounting)for your accessories.Altenator,P/S,A/C,etc.You'll either have a short W/P set up or a long W/P setup.

So, if I'm planning to get new brackets from March including the pump, which one should I choose? what does it depend of?

Short water pump cars have the alternator (and it's wiring) on the drivers side.

In '69, Chevy went to the long pump on the passenger side on all cars ecxept corvette. This was also when they went to head with the accessory holes.

Prior to '69, the alternator hung way off the drivers side of the engine, on a bracket attached to the exhaust mainfold. 69 and up, the alternator bolts straight into the accessory hole on the head. It tucks in real close, and completely in front of the engine.

You pretty much will have to let the location of the wiring dictate which setup you use. What year is your car?

It's a '67 RS, but I have a Painless harness that will be installed shortly when the engine goes in...Thans for the help!