[Los_Control]

No expert here, I am going to guess what you really want to start with is gearing in the rear end.
Almost ashamed to say, I have a 1949 dodge with a flathead 6, 3 speed on the tree and stock 4:10 gears in the rear. It will do 45-50 mph.


I know my 218 6 cyl if I installed a 3:73 gear ratio, it will run 65-70 mph and have plenty of torque to pull a hill in 3rd gear on the freeway. If I used 3:55 gears, just a little to tall and on hills not enough torque and going to have to down shift ... not ideal on the freeway at slow 65 mph with a 3 speed and traffic running 80 mph.


Hope you see where I am going here, 4:10 is to low, 3:55 is to high, 3:73 is just right ... listen to the smart people here guiding you to a rear end change, and learn what gear ratio works for them with the torque of your engine. Then later swap the transmission if you think you want to.



For me, is as simple to buy a ford explorer rear end from a junk yard, spring perches from tractor supply and a new ujoint ... for you is different because you want to keep the big bolt pattern on the wheels ... I am just ignorant on the ways to swap the rear end in a 1940 Ford truck and use original wheels.