When we ran a full service chassis fab/turn-key hot rod shop a number of years ago, it was always useful to have threaded things around to weld into the frame for some kind of mount, etc.
Here’s a list of all our threaded things for your reference:
Spring Mount Plate
Product Details
Brand: Welder Series
MPN: 21271
Thickness: 3/8"
4-1/4" x 3-1/4" x 3/8" thick. Available for 1-3/4" and 2" wide springs.
Four holes, 3/8″ diameter on 3-3/8″ x 2-1/4″ centers. The center hole is 7/16″ diameter.