42RH / A-500 in a Duster with pictures.

You’ll find There are a few threads on this swap with a quick search. Here’s my 727 to 46rh swap thread.
727 46RH conversion coming

In all the swaps I’ve seen to an A500 or 518 they’ve all required the upper crossmember to be cut and reconfigured or replaced somehow. The tunnel also usually requires some hammer dimpling clearance got where the trans tail shaft overdrive section bolts to the main housing. The UScartool crossmember works great but some guys have made their own to save a few bucks. I did tons of research before I decided to jump in but was well worth it I think.