Drive Shaft Lenght

Hi
I'll be replacing the 7 1/4 rear with an 8 3/4 in my '70 318 automatic (904) Barracuda soon.
Does anyone know what length driveshaft I'll need?
Thanks

It’s best to measure and have your old driveshaft cut to length or have one made.

7-1/4 diff is 10.09" from pinion to axle centerline. 8-3/4 is 12.35" . Therefore you need to shorten the driveshaft by 2.26"

That is unless someone has shortened or lengthened the car, in which case you better measure it !!!

Yep…I would do the swap and then measure after the new rear is in place.

measure twice, cut once

I like Stanton's approach BUTTTTTT

Thanks for the replies.
I have a few extra drive shafts on hand and was looking for an overall length to fit my soon to be combination.
The car's going to end up with a 440/727 (and maybe a gear vendors) so, being a cheap b*stard, I'm hoping to use something in my stash.

i always measure three times, cut five times, and it's STILL too short..........

That's cause you're butt cracks not showing, has to be to get an accurate measurement ask any construction guy 🤣🤣

I like to shove the tranny yoke in all the way and then slide it out 3/4 inch,.750, and use that for the center to center of the u joint distances that is with all the weight of the vehicle on the tires on the ground or something acting as the ground like jack stand under the rear end housing and front lower control arms

What Cab says. Most engagement of the output shaft you can get without bottoming.
With the way tolerances stack up, especially on a build-up of an old car, driveshaft's one of the last things to do.

100%

absolutely the correct way.