I think the easiest thing to do for the 4WD system is use the parts from the rereleased Monogram '81 Bronco, since it's the same scale. However you'd have to mock up the parts on the chassis first to make sure that they will work with the AMT kit.
They're not the same scale. The Monogram kit is 1/24 scale. The AMT F-150 (the kit this is based on) is 1/25. The Monogram Bronco TTB is pretty lousy anyway. The AMT/MPC Bronco has a much better detailed TTB under it judging from the photos I've seen of it. I'm going to try to score the AMT Bronco soon and do one of these conversions for myself.