Do I need new driveshafts when swapping to an overdrive transmission behind an LS?

Almost always. Speedway notes a transmission swap nearly always means a custom-length driveshaft: overdrive units are a different length (a 700R4 is about 30 in, longer than a TH350), and engine position usually changes in a swap.

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.

Front slip yoke by transmission: | Yoke | Fits | |---|---| | GM 27-spline (1.5 in seal) | TH350, Powerglide, 700R4, 200-4R, 4L60, 4L60E, 4L65E, 4L70E, Muncie, Saginaw, T-5, T-10, T-56 | | GM 32-spline | TH400 (no threaded hole in the output shaft), Muncie M22, Super T10, 4L80E, 4L85E | | 31-spline (Ford C6 style) | Tremec TKO, TKX, T-56 Magnum, Magnum-F, TR-6060 |

Speedway's custom-length kits are rated up to 400 hp (small kit), 600 hp (3 in steel kits) and 1,200 hp (chromoly).