rear end swap drive shaft lenth

I don't know the length but I do know it will need to be 1/2" shorter than the original cause the 8-1/4" rearend yoke centerline is right at 1/2" longer than a 7-1/4's centerline. If you know the length of the original just shorten it 1/2" and you should be fine. If you don't know the length of the original and have a yoke to slip into the transmission do that then slide it back out 1" and measure center to center of the U-joint mounting points with the car sitting at ride height and that's how long the shaft has to be.