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:
End flange for transmission mount kits
Product Details
Brand: Welder Series
MPN: 132508
Country/Region of Manufacture: Canada
Thickness: 3/16"
Bolt Size: 3/8"
This is the flange used in our universal transmission mount kits. 3/16" pickled & oiled mild steel. Bolts are on 2-3/4" centers.