Late update, just in case this would help anyone: I removed and labeled all the wiring which serviced the Leece neville alternator, the rectifier and the regulator. I left everything else as I dont want to completely rewire the truck. I ended up purchasing a 60 amp 20si alternator made by Dixie Electric (found a good NOS unit on Ebay). It suited my needs because it can be fitted with a readily available large 2-groove 7/8=1" pulley (Caterpillar spec), so that the original GMC dual belt setup can be retained. I also purchased a heavy duty longer tension arm made by Jimco. I ground out some notches in the bracket half which remained on the engine block to accommodate mounting legs of the new alternator, then slid a hardened 7/16 threaded rod through all of it to hold the alt in place at the bottom. The notches keep the alt from moving forward or back. I re-used the heavy duty 100 amp ammeter which came with the truck. It works fine, even though I know most people say to use a voltmeter with alt. I wired it just as it had been, using 8 gauge (not sure, it might be 10 gauge) directly from the alternator, then back out to one side of an emergency kill switch on the firewall, which is designed to cut power from the battery. This has worked for me, not saying there wouldn't be many more ways to do it!
