NAPCO is the company that supplied the 4 wheel drive system for GM. It consisted of 7 leaf front springs, front axle, transfer case and lift blocks for the rear. Before 4 wheel drive was available from the factory, the dealers would install the NAPCO system at the dealership on 2 wheel drive trucks. Quite easy, just drill a few holes for the transfer case and handle. I believe in 57 it became a factory installed system. I have never heard of a swivel frame, it sounds awesome! I am guessing that the swivel frame and NAPCO were just options on your truck, they don't have anything to do with one another as far as I know. What would you need a swivel frame for?
Anyway, if you are interested in NAPCOS, check out my project. I took a 55.2 rear wheel drive truck and installed a NAPCO system from a 58 Suburban. The 58 also came with a rare option for a 10,000lb PTO winch that runs of the transfer case. I believe it was a forest service truck. It is totally radical. Love it. There are like 5 drive shafts in my truck.
55.2 with 58 NAPCO 4x4 system 
