What Cosmo said 110%. Get a donor truck 6 lug diff 55-72 with what ratio you like. Get a driveshaft with it and get it modified. Get rid of Torque tube. Go open. Then if anything goes wrong, the parts are easier to find, install and more reliable. Way cheaper and DIY. Just study google and here for what diff has near WMS-WMS and what it takes at tranny. Lots have done it, lots of info. You can buy 10 diffs for that money.
If Cosmo says 3.90 works, he knows. I would go higher, which is a lower number.
BTW I have a nice 1953 AMI 120E.
