Could do what I did, I picked the shortest available post and then cut it down to size needed for my HF lathe - Although that'd probably be way too short for Powermatic, so you could just grab the longest available (I believe they are all priced the same?) and then cut down to size if needed. posts cut with a hacksaw rather easily. However, after trying the straight and S-curve rests on mine, I came to find I really prefer the Oneway rests I had been using - the round bar sorby rests just aren't quite right.. So bear that in mind before going whole hog on the Sorby modular system - Might want to try using a round tool rest (could have one welded up from 1 inch bar stock?) to see if you like it.
*also - I thought they gave the length of the tool rest posts , and if they do, then measure one of your powermatic posts?