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:
Mustang II Crossmember Boxing Plate
Product Details
Brand: Welder Series
MPN: 143072
Country/Region of Manufacture: Canada
Thickness: 10 ga
For Mustang II crossmember. 6-1/2" x 3-1/2", 10 ga mild steel.