OK, I got to talk to the builder John at CRT about a 518/46RH https://www.coperacingtrans.com/?product=crt-518-hd-1-transmission When I asked, he did not think that the small block TC should be a problem - he said he's "built many of them" for heavy towing applications and can supply the BB adapter, core, TC, etc as a full kit. He also recommended not to use pressure switches, but only toggles on the dash for control.
