What driveshaft works for a 700R4 conversion in a 71/72 C10?

A custom-length shaft with the right yoke.

How to measure (Speedway's method):

1. Put the rear axle at ride height — not hanging with the car in the air.
2. Measure from the flat face of the rear yoke (the center of the U-joint once installed)…
3. …to the seal surface on the back of the transmission tailshaft housing — not to the end of the output shaft, which makes the number come out short.
4. That one number, plus the correct front yoke and U-joints, is what a custom driveshaft is built to.

Yoke: the 700R4 takes the GM 27-spline slip yoke. Speedway's custom-length kits are rated up to 400 hp (small kit), 600 hp (3 in steel kits) and 1,200 hp (chromoly). The 700R4 is longer than a TH350 (about 30 in depending on tailshaft), so the old shaft is usually too long.