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:
Flat Crossmember Center Plate
Product Details
Brand: Welder Series
Thickness: 3/16"
3/16" P&O mild steel, 5-5/16" x 1-1/4". Used in our flat front crossmember.