Sam
I have the same motors and useing 66, 42, 18, inch screws. I am deciding on scrrew size and pitch myself.
I would suggest this
great read .
http://www.cnczone.com/forums/showthread.php?t=5866 It is a well written and very informative article. This will give you a good idea of the screw you need. After this check out the specs on the different screws. They all have suggested max speeds.
I will be watching this thread for other suggestions and answers to the other questions I was about to ask about ballscrews.