GM to mopar adapter plate

I'm pretty sure you have that backwards. That looks like an adapter to put a Mopar 833 gearbox up to a Chev / GM bell

Similar adapters were actually used OEM to install Mopar 833 OD boxes behind Chev engines, even some in 4x4s, although some of those Mopar boxes had special cases that bolted directly to a GM bell, with a GM pattern input shaft.