42RH SWAP

They (could) have a lesser clutch plates in the OD and a few other things I recall.
I daily drive with a non-lockup 518 in my '73 Dart.
No issues with the non-lockup and driving slower than the (stock) convertor's stall speed.

I can 'switch on' OD at anytime the trans is in 3rd.
I can also coast down to say 20-25mph and flip the OD-switch and have the engine 'idle' at around 7-800 rpm while still moving nicely. Not the most efficient way to drive but it sure is 'quiet'.