Hi, How do you tell the difference between a GMC 6-71 blower and a 8-71 blower. I know theres a size differnce, but how much? Thanks

The 6V blower has two sets of ribs on the case where the long bolts attach the blower to the block.
The 8V has three sets of ribs.
The inline 6 has three ribs and attaches with bolts thru the flange at the base of the sides. And one side has a lip on it where it kind of hangs on the block.
Don't forget that the bearings on the end plates need an oil supply and a drain, too.
Hope this helps.
Frank