How far you press the flange is entirely dependent on which pulley you use. I have used a factory V8 pulley that has a shorter offset than the factory 6-cylinder pulley. That makes room for a 6-blade truck fan that you can remove without removing the radiator first. You'll find that if you use a 16-inch fan, you'll have to drop the radiator several inches for the fan to clear the lower hose connection.
Later, I added AC and rather than trying to drive the fan, alternator and AC compressor off the one water pump/fan pulley, I got a 2-shiv crank pulley and used a V8 2-shiv water pump pulley for an air-conditioned V8 with short shaft water pump. Whichever V8 pulley you use, you will have to press the flange an additional 1.1 inches onto the pump shaft. Don't use my figures but measure it yourself for your application. The distance that matters is from the block mounting surface to the pump's pulley mounting surface. That dimension has to make the shivs on the crank and water pump line up exactly.
If you are using an after-market short shaft pulley then its offset will determine how far to press. You get one chance to get it right because it's really hard to press it back out with a regular shop press (at least with my 20-ton) and a puller won't move it in my experience. If you go too far you'll have to shim it back out.
