Ironic you mention this; I traded all of my 56 series 4 speed stuff to OGREInc for a freshly rebuilt 63 series 5 speed last month. Beyond it being stronger it's also possible to assemble a mid close ratio box by using the alternate 2nd gear from a Z20 powered 81-82 510. Th3 alternate gear is 1.9 versus the stock 2.017 in the B210, this may not seem like a lot but when you're buzzing the motor to 8200 it means the motor drops 400 fewer rpm. On my motor the 400 rpm keeps the motor above the point where it steeply falls off, it's worth something like 8 horsepower.

After the 63 series is in I'll be selling of the 60 series because the 63 isn't likely to ever break.