Trying to put the stock gears back in my car. The drive shaft that has been in there is from a Dodge truck I believe. I have drag gears in it right now, but I just want to put the car back to stock. Long story, but looking for a drive shaft because the yoke does not fit the rear end. I measured the existing shaft and it was 52". Found one online at 51.5". Asked the guy where it came from and he said a 904 auto with a 7.25 rear. I have an 8.75 rear with a 727 auto. I'm guessing I can't get this drive shaft? Any help would be great!

if i read your post correctly, you are changing just the GEARS inside of the case.
if that's correct why do you need a driveshaft? the gears are internal. if the U-joints will be a different size after the gear change they make special U-joints with 2 different sized ends for situations like this.

So, to make life easier I wanted to keep the drive shaft in there. I have the stock rear end and bought a 7290 large u joint axle yoke kit. When I attempted to install, the existing u joints on the drive shaft were to large for the yoke. I bought new 7290 u joints that did fit with the 7290 yoke, but they are a hair smaller than the holes on the rear end of the drive shaft where the u joint fits. I can't think of any solution except to get a new drive shaft that actually fits in regard to the u joints.

Whole different rear end, not swapping gears. taking one rear end out and swapping with stock one

change the pinion yoke on the 8-3/4 or use a conversion u-joint

2 common ujoint sizes are the 7260 and the 7290 & the width of em from side to side/C clip to c clip is 2&1/8 on the 7260 and 2&5/8" on the 7290 plus the cap dia is slightly smaller on the 7260 than the dia of the 7290 caps. with the rear axle on stands & the front yoke butted all the way into the trans then measure the C to C distance & subtract 3/4-7/8 from that number & that is the C to C of the correct shaft you need (compare that # to what you have & to what that guy wants to sell you). As you know the 727 and 904 use different front yokes.