I just stuck it in and hoped for the best, I really wanted to see if the piston seal conversion I did was going to work, which it seems to have, the ram body seemed to fit snuggly in the HW90 rear guide and there was probably only about 10mm of the rear guide sticking out the cylinder, so just put it all in the big Sach clamp I use and then put the cross pin in after a couple of turns, I remember you shouldn't alter the piston ram rod on the Theoben drop in rams, so didn't touch this one either, it all seems to work well, not sure if any preload will damage the ram or not.
Pete