How do you measure for a driveshaft with a 700R4 in a 67-72?

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: a 700R4 uses the GM 27-spline slip yoke (1.5 in seal). Speedway's custom-length kits are rated up to 400 hp (small kit), 600 hp (3 in steel kits) and 1,200 hp (chromoly).

A 700R4 is about 30 in long (depending on the tailshaft housing), longer than a TH350, so an old TH350 driveshaft is usually too long. Measure after the engine and transmission are mounted in their final position.