Charlie, S+W does do custom stuff and should be able to make one for you. 1 800 523 3353
http://www.swracecars.com/
The drawings above should help them.
Tell them to drill for BOP and Chevy pattern(in case you come across a tranny deal that has chev pattern). Use 61-64 Pontiac big car starter(3.25" pilot)(Butler sells an IMI mini starter for 61-64) or chevy inline six.(smaller pilot)
Pilot plate should be a snug fit to starter. The hole in mid plate can be a bit larger to allow positioning pilot plate for best starter to ring gear clearance. (I clamped mine and tapped with a hammer till I was in middle of clearance range then welded it to engine plate)