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:
Alternator Support Plate
Product Details
Brand: Welder Series
Thickness: 3/16"
Formed, 2-1/4" x 1-1/2" with a 3/8" hole in the 1-1/2" side.