I am redoing a 1955 1st series 3600. I plan this to be a restored part time working truck 200 miles/month or so. I also would like to use it to tow my car trailer. I am installing a Dana 60 rear end out of a (pardon me) F250 which I believe has 373? gears. I also have a 1957 261 engine with an 848 head I plan to install. The transmission I plan to install is a NP 540 (not sure if it is 540,541 or 542) 5 speed with 1:1 in 5th. I would rather run a SM420 but think the NP would give me a better range for towing. I find the SM420 has a gap between 3rd and 4th Would appreciate any suggestions, comments or opinions.