Trying to find the correct length(center to center on the u joints) and diameter a driveshaft would be for a 68 Dart GTS with a 383 4 speed and 8 3/4" rearend, and should it be a 7260 or 7290 yoke?

If the inside to inside distance on the ujoint yoke flange is 2&1/8" it would take a 7260 ujoint. 2&5/8"-7290

He wants to know how long the driveshaft should be from yoke centerline to yoke centerline and the diameter of the tubing. Not which U joint he needs

Both the Factory Parts Book & Service Manual give the length as 51.53", the diameter is 3.0" and the u-joints are 7290.

You offered no relevent info so your post is worthless. He was not getting any action & alot of views so I threw something out there to maybe generate some interest.

OP did ask for U-joint info.
Robert told him how to measure for U-joint size.
Twodoor gave a comprehensive answer from the FSM.

Thanks for the input everyone, I got all of the answers I needed !

however, if it were me, i would stuff the yoke into the transmission until it bottoms, then pull it out 3/4-1", then measure the center to center distance on the yokes with the weight of the car on the rear end to get the shaft length necessary for YOUR car.
over the years, i have found the factory listed length to be somewhat short on more than a few installations, leading to vibrations that are hard to pin down.
just my experience, your mileage will vary.