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 Mount Plate
Product Details
Brand: Welder Series
MPN: 238211S
Thickness: 3/16"
All holes are 3/8". For small block Chev.