Installing a 8.5, do I have to change driveshaft?

[Call Her Bu]

im looking to get a 8.5 installed in the rear end of an 81 Malibu, I was wondering if the driveshaft was gonna need to be shortened? I'm installing a 6.0 LS with a 4L60 trans. Any advice or opinions will be helpful. Thanks.

 

[truracer20]

I think it's more than likely you will need the driveshaft shortened. It really depends on what transmission your current driveshaft is for.
But a TH350 and 4L60 are within 1/8" of each other, So in theory if you could mount the engine 1" farther forward(measured at the transmission to block mounting surface) of stock then the driveshaft would be fine, although you have to verify this to be exactly sure. Your issue then would be the transmission mount and the hole in the crossmember would be off by 1". This will also cause interference between the crossmember and transmission case/pan. You would have to build a custom crossmember.

Honestly I would install the engine to use a 700R4 crossmember and have a driveshaft made or get the original shortened.

 

[oldtinsmith]

If you were to replace the 7.5" rear with the 8.5" rear, you would need to shorten the 7.5" driveshaft 1" to fit. The 8.5" rear requires a 1" shorter driveshaft from the factory in the GN, H/O, and the 442. ... Wait until you're done with your 6.0 Ls engine and trans install and then measure for the driveshaft. The Ls engine may require a slight set back.

Doug