I did some digging and I was mistaken. The D50 TTBs DO use the larger 1350 outer joints, however they are not interchangeable with the D44 knuckles. You would have to do alot of fab work to make it fit.
The D44s use the 760x for the outers and inner joint.
The D50s use the 1350 for the outers and inner joint.
From personal experience I know the D50 third member and inner stub assembly are 100% bolt in for the D44 TTB.
'81 F250 4wd 302-ZF5-NP205
'81 F150 2wd 302-AOD