Need Help With Driveshaft Issue

Okay guys, I need some help. I'm doing the last few things to get my Dart on the road after my big brake, mini-tub, spring relocation, tires & wheels project. I went to put the driveshaft in today and found a problem I did not anticipate. I have two driveshafts, and neither one will fit.

I bought this car last fall and for now I'm leaving the 318/904 in it to drive this summer. This car had 7 1/4 rear, but I just installed an 8 3/4 rear from my '69 donor car. It's now has a 904 trans with a 8 3/4 rear end. I have two driveshafts, and neither one fits.

One shaft is from the 904/7.25 car and it's too long and the u-joint is too small for the rear end yoke. The other shaft is from my old '69 Dart 440/727/8.75 donor car. It is too short. I didn't check, but I think the shaft yoke is too big the the 904?

What do I need to do? Am I missing something obvious?

Thanks for your help!