Pictured are the flat perch commonly used in tri-5 Chevy, Mopar, Mustang, Caltracs, and many more applications.
If you are making your own housing these may be of help. They fit a 3" tube stock diameter.
These perches are common on the following:
1955 1956 1957 Chevy Tri 5 - 2 1/4" Perch
Caltrac - Boxed in for strength