the 7 1/4" from my M was 61" drum to drum. I've heard the 8 1/4" are about 1/2" narrower. either way, they're about the same width as a '68-70 B body rear.

1976 Spinnaker White Plymouth Duster, /6 A833OD
1986 Silver/Twilight Blue Chrysler 5th Ave HotRod **SOLD!***
2011 Toxic Orange Dodge Charger R/T
2017 Grand Cherokee Overland
2014 Jeep Cherokee Latitude (holy crap, my daughter is driving)