This truck is going to have 8-lug wheels and full floating rear hubs. I want to run 16 inch wheels with real light truck tires and I don't think the Cadillac IFS would be the right way to do that.
I think either the original straight axle or the '63 to '87 design IFS are my most likely choices. A later modified straight axle (Grigg style) is probably the next direction I'd consider with straight axle type four-wheel-drive another possible contender.