If you buy the housing already cut and turned it may also come with the perches on. They will probably be able to set it up just by the info you give them (springs, shackle reversal, cruiser model, etc)
If you do your own the easiest way to set up the spring perches (if you haven't done a bunch of them) is to cut the old perches and anything else you don't need off and clean it up, then roll the axle under the rig. Set the new perches on the axle and jack it up to the springs. Then rotate the pinion to the angle you want and make sure you jack it up so the springs are taking the weight and make sure the pinion angle is still how you want it. When everything looks good tack the plates. Then you can drop it back down and finish weld them.
Then cut and turn the knuckles to reset the caster.
A coworker has a sprung over 60 on 35's, i'll ask him how they fit and if he thinks 37's will fit. (it may be for sale too, not sure where you're located. it needs a shackle reversal and it has a bad rod knock, so new motor too. really could use a V8 for the size rig)
