I own a 68 chevelle with a 69 police engine. it has the 69 long water pump. I wish to switch back to the 68 and earlier short water pump. I want to make the engine at least look like an original 68 engine. Do the two both sufficiently flow water? and what is involved in the switch? I know the fan shrouds and hoses are different...but are the pullies on the crank power streering and alt different? and is the radiator different?

If the cost is too great I may put it off until the frame off resto planned for after College and get a house, job, etc.

Andy,
The following assumes that you have a big block...
I have never heard anyone say that either pump flows better than the other. If you switch to the short pump, you will need new waterpump and crank pullies. The crank pulley is actually two pullies if you have power steering, and two different pullies if you have power steering with A/C. You will also need the alternator brackets, upper and lower, that mount to the driver's side of engine. Then there is the power steering setup for the 68 and earlier big blocks. This includes two brackets, the pump with pulley, the remote reservoir, and related hoses. No need to change alternator pulley or radiator. No need to change the shroud either, as long as it is a '68 unit. And for correctness, you will need a new fan as well.
This may all sound daunting, but I can assure you, the switch is very easy to do, albeit a bit pricey. The parts are all readily available aftermarket. Original GM stuff may take a little longer to locate, but in my opinion the hunt is half the fun. Good luck...
Patrick