Iso spring perches were made with a 1 1/2" center hole.

The MP perch with a 5/8" center hole was originally a Dakota application (3/8" spring center bolt). Installing them on a passenger car 5/16" center bolt is a non-issue because this bolt locates the shock plate, and the shock plate locates the rearend housing via U-bolts tight against both sides of the spring perch.

I carry special 5/16" center bolt spacers, however.