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).
mine is a 1954 GMC 350 (2ton) i cant believe i forgot to add in my truck!!!
thank you! yes im looking for around 70 without screaming not that ill be going that fast but to have the option makes life 1000X easier.
