See same post in engines section this board for how and why. Speedometer will be affected when using over drive because trans is in backward and you are overdriving it after the speedo gear. Second gear needs to be shiftable to take it out of OD. The first reverse slider can be removed to reduce rotating wait if so desired.

Been there, Done that, Hope to live long enough to do it again.
Big Bill
I.I.# 4698