WD,
Aren't your hinge plates tapped either!?
The tolerance between the hinge area and the actual hinge is very small. A 3/8's bolt needs to be used 3/4" long and it only leaves +/- 1/8" before the hinge pin. I do not think welding a nut will allow you the clearance or give you the strength needed at this point.
Of all people tell me you do not have a 3/8's 4-sided tap!
Maybe it is a good thing I posted this?
Tell you what, if any of you don't have the tools, the ability/knowledge, or just plain to nervous to attempt this, send me just the "U" shaped brackets for your hinges and I will get them sent back out the day after I receive them! You just pay the shipping, I'll do the tapping!
DV