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:
Transmission Adapter Tab
Product Details
Brand: Welder Series
MPN: 35012
Thickness: 3/16"
Hole size: 3/8"
Often used with the #WS35010 adapter plate. 2 holes, 3/8" dia on 4" centers.
I've used this plate for the tabs with the 3/8" holes to make a multi-angle adjuster.