the TTB is a joke on leaf springs, how much twist do you think you can get out of a tapered leaf spring. this is basically what the Dana 50 does when suspension is moving up/down.
the beams are short on the leaf TTB. just put an older F150 with coil TTB and an F250 TTB 44 or 50 and try it. leafs were meant to be under solid axle!
I bought a brand new 96 F250 loaded truck and the only problem the truck had was the horrible handling of the 50 TTB on the road! no amount of alignment and adjustments coudl keep tires on it or make it go straight down the road. looking back I wish I would have put a solid axle in it and the rear blocks and kept the truck. I just couldn't bear to tear it apart while it was under warranty.
look for the parts guy who can still read a paper catalogue , he will find you the right stuff. my local parts guy , now co/owner of the local Napa store has been in the business since 1986. I just ask for him when I call for parts, he even knows the outside sources for my older vehicles when Napa doesn't show anything available.
if you have a Dana 50 and you aren't fussy on pavement handling stick with it, their strong point is the external full snap rings on the Ujoints. they will take a lot of abuse if kept greased, stronger in my opinion than the dana 60 internal Cclip for shock loads like plowing or offroading. very limited articulation is a limitation.