4l60e To Th-400 ????
The driveshaft length should be correct. However, install it into the transmission and slide it back into the rear axle yoke. There should be a minimum of 7/16" of lateral movement before the driveshaft bottoms out into the transmission when pulled forward from the axle.
Several vendors here at Tech manufacture those crossmembers... try BMR, Spohn, TCI and Yank.
Hope that helps,
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