drive line/u-joint help

[dukcaln]

so I swapped motors and transmissions this week. I ditched the TH350 and replaced with a TH400. Same size motor, sb350. I found a complete drive line out of a 72 that had a factory TH400 and figured it will match right up.Which it did other than the u-joints on the factory TH400 are larger than the ones on mine that I removed.

So the situation is the rear end has the smaller spline for the smaller u-joint. I was told napa auto sales a u-joint that has the two sizes built into one u-joint. Anybody ever done this?
I thought I might be able to split the drive lines and use the back half of mine with the front of the factory th400 section but it looks like the spline with the carrier bearings are different?

Hope I am clear with waht I am trying to explain without pics