It's not that tough a conversion, really. The rear rails do look a lot like the old Mopes, and the L-body pickups (Rampage/Scamp) did just fine with leaf springs.
The front suspension from any F-body or variant (M and J-cars) will work; I'd just make mounting tabs for the K and use aluminum or poly bushings instead of welding it solid. As far as the rear goes, if memory serves the van axle is actually wider at the lug pattern than most Mopar RWD axles.
I think you'd need to do something about the fuel tank, though... isn't it right in the path of the driveshaft?
BTW, this conversion process is nearly identical for all the K-based Mopars--Shadow/Sundance, Daytona/LeBaron, Caravelle/NYer/400/600/Aries/Reliant, and the vans.