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:
Clamp Plate Spacer
Product Details
Brand: Welder Series
MPN: 15703
Thickness: 1/4"
Hole size: 9/16"
3-1/2" x 1-3/4" x 1/4" thick. Hole is 9/16".