Richmond has a 5 speed rated to 600 ft lbs. It's like $3,500, but it comes with a really nice Long shifter. Still, big bucks. And I haven't really heard of anyone who has one to find out how they shift and run.
Still waiting for the 5 speed Muncie. . . and waiting. . .
For clutches, the centerforce dual friction (CFDF) clutch works well, in my experience, and the other one is the Mcleod twin disk clutch. CDN SS runs that clutch behind a 630 hp 427 bb.