Please tell us what exact truck you are working on.
One option to keep your SM420 with park brake (and assuming working and fairly accurate speedo) is to use a ranger torque splitter (overdrive) that goes in front of the trans, between it and the bellhousing.(Advance adapters sells them new. ) That is what I have in my 58 t-ton. That said, it will do 70 on the freeway, no problem. (don't really want to go much faster than that).
