I am in the middle of building a M101A1 myself. Just swapped out the axle for a 6 lug 5200lbs with powder coated steel rims and BFG AT's 265/75/16 E's. I ordered the axle from ABC trailer and it was around $240 shipped. Your spring perch's should measure 43.25 at the centers. My axle bolted right up like a glove. I am working on a hard lid right now myself. I am going the 1/4 marine plywood covered in 24oz fiberglass. Steel would be to darn heavy and aluminum is just too much $$