Remember that the minor diameter of the bolt is where the strength is, the shank part, not the thread. (OTOH, for a 1/2 NF that looks like .45" or so, which ain't bad.)
However, I would strongly consider 1/4" or 3/8" plate. 1/8" plate with just four bolts will want to deform when you start sliding a coupla tons of truck on levers that are what, two feet long? Think of the forces being applied to it.
Even if you bolt around the entire circumference of the plate, it'd still flex a bit which will eventually break the welds, which is bad juju
If it were me, I'd use a nice thick plate on the outside (1/4"+) and yeah, 1/2" bolts, maybe six depending on size/shape of the plate, but just fender washers on the inside. The bolts should only be carrying the load of holding the plate to the frame; the plate should be carrying the torsional load. Given that, fine thread bolts might be a good idea, though threadlocker is a good idea too.
If you DID do a plate on the inside, you could tap the holes, though after doing so I'd thread a nut down and weld it to the plate. Overkill with fasteners is almost always A Good Thing
