OK, I spoke to both Transtech (Iowa) and Phoenix Transmission Products (Texas) and both said they did not have a product for a heavy vehicle.
Transtech said the welded-on bell will not accept a large TC - needed for the 413 RV, and Phoenix only builds A-500 mods, not 518.
JW Performance Transmission does not have an Ultrabell to do it.

So, it looks like the Wilcap adapter and finding a 47RH is the only solution.

Along this line, has anyone heard of or used Hughes TCs?
http://www.hughesperformance.com/xtm-xfm/
they said they have a 24XTM for the 727, and a 15LXTM for the 47RH, both for towing etc.