I plan on converting my 70 Cutlass S to a 4L80E. What is the correct way to measure? It is the suspension fully extended correct? Does anybody have the actual length needed with the 12 bolt O axle along with needed U joints? I know the yoke needs to be the TH400/4L80E 32 spline yoke. I will need a whole different driveshaft, my current is the factory TH350 with the rubber connector holding the two pieces together.

 

You want to measure with weight on the wheels. The yoke needs to have 3/4 of an inch freeplay and measure center to center of the ujoint cup holes between the yoke and the rear diff.

 

We had one made for my son’s 64 C10 pickup. Called the drive shaft shop. They wanted the measurement from the center of the rear u-joint (which is the strap mounting surface of the yoke) to the end of the trans tail shaft housing. Turned out perfect. Call your driveline shop, they will walk you through it.

 

Thanks makes sense to follow their lead on measurements.

 

If you are looking for a shop, I have had two made from by PST. They have a quick turn around and are helpful on the phone.
https://pstds.com/
I think the turn around time was about a week from the time I called until the shaft was shipped. Both were 4" aluminum shafts built from yoke to yoke. They supplied the trans and rear diff yokes and my measurments were seal to seal. Whatever supplier you use, be sure to discuss critical speed and shaft diameter/ material with them.
Here is the driveshaft they made for my Vista.

 

Yeah I want something strong that won't rattle out my teeth.

 

Dennys Driveshafts
See the measurement sheet
https://www.dennysdriveshaft.com