I machined a plate to mount my 12 Volt electric winch on the tongue of my little trailer.

The mounting studs from the original plate are 3/8 propietary shoulder bolts. I drilled the 3/8 flat bar that I had laying around and machined enough room on the bottom, to accept the lock washer and about half of the nut. That allowed me to use the same bolts, even though the flat bar is thicker than the original plate.

I'm going to weld some 3 in x 1/8 flat bar to each side of the bottom of the plate that I made.

When I mount it to my 3 in. trailer tongue will 3/8 Grade 8 be sufficient to bolt through the plate and the tongue, from the sides?

I will use this 6000 lb (on a rolling load) winch to pull my 500 lb stacker onto the trailer.