I just spoke to precision torque converters https://gopnh.com/ They will build a lockup converter for an A-500 or A-518 to just about any spec you want. They said the can give up to in the area of 5500rpm stall and with a triple disc converter clutch and a billet cover.
Wow, things have changed, they did say they could make it smaller, but it'd have to be a dual clutch, & that would compromise how much it'd stall at that size. Guess demand & will means it's only a matter of $$$$ now... @92b