Is the attached pic below (taken from my 1947-54 FAM) the diagram you are referring to???
If your rear mount is like this, that long bolt needs to have the sleeve (spacer) and the "bolt lock" plate to install/secure the mount and bellhousing correctly to the crossmember.
If my info is inaccurate, am sure fellow Bolters with more knowledge will come along and offer more info.
Dan
~ Dan 1951 Chevy 3 window 3100 Follow this story in the DITY Gallery "My Grandpa Carl's Truck and How it Became Mine" 1966 Chevelle (Wife's Hot Rod) | 2013 Chevy Silverado (Current daily driver) US Army MSG Retired (1977-1998) | Com Fac Maint Lead Tech Retired (1998-2021)