I have been playing with BW overdrives for 50 years and have R10 and R11 back sections in my store room. I also have some SM420s. I have toyed with the idea, but I see no way of doing it without a lot of machine work. I think if I were to try to do it, I would start with a 4 speed with an open drive shaft and fabricate a nose section with with an input bearing on a shaft with a seal that would accept a companion flange. The nose section would have to include means to securely mount it to a cross member. I think making the OD unit into an divorced unit would be more doable than trying to attach the OD to a SM420. And then it would work behind any transmission with an open drive shaft.