2wd G56a transmissions are pretty scarce, but I finally found one through one of my parts places. It came out of a wrecked 2011 Ram with only 17,000 miles on it.
It came in today on a pallet, it's all wrapped up still, so here is a list of the knowns and unknowns.
The G56 is physically longer and taller than a NV4500, so there will probably be some clearance issues. Net research says I'll have to cut crossmember and hammer floor.
The G56 will (probably) bolt to the NV4500 adapter. The G56 uses an integral bellhousing.
The G56 is fixed yoke vs. my current slip.
I'm not sure yet if the stock clutch and flywheel will work.
Speedo pickup may or may not work. I found a writeup online detailing how to use rear axle sensor for a speedo sensor.