If you have been paying attention JP recently upgraded the axles under their project M715. They used a '85-'96 Ford dual rear wheel highpinion D-60 front axle and it is a direct spring-over bolt in for our trucks. Measuring at 77" + or - it works well with surplus Humvee wheels on M trucks. They also used a D-70 rear from a Chebby dually (not cab and chassis) truck that measures 73" wide.

The problem with this rear is that you still have to move the spring perches. I have found a better solution. A Chebby G30-G.M.C.G3500 van with dual rear wheels features a 73 1/2" wide D-70 and more importantly SHARES THE EXACT SPRING PERCH WIDTH AS OUR TRUCKS!!!!! It is a bolt in and as an added benefit is wide enough to allow use of the chevy drums with their parking brake system.
Put one under my truck this afternoon and reused all the van stuff like u-bolts and anchor plates. One shock mount must be moved as the van shocks are staggered(one in front of the axle and one behind) and the factory jeep rear brake line needs to be lengthened because the van flex hose is on the passenger side. Simple swap.

Jp used 14bolt GM hubs to widen their rear out to match the front axle. This works great if you want to run discs on the rear, however the rear disc parking brakes sucked when new and installed on a passenger car. I doubt they would hold a Kaiser with big tires. I plan to leave my truck drum rear and run 1 1/2" spacers to get my width right.