Sam, I'd use a nut rather than a bolt. Then, see what will fit on there (nut AND socket) before you weld. You'll have to use a high heat setting on the MIG, and pull away before you melt thru the outside hex. But this is also great directed heat in the bolt.

I wish MB made less use of the allen bolts. I disassembled 2 transmissions this summer and made 1 good one. The B1 reaction pin retainer has a M12 allen hex in it. I had a huge struggle with BOTH of them. The only thing that worked was striking the periphery of the bolt head with a cold chisel sharp enough to catch, but dull enough to not cut easily until it bit in, then continue to strike until that part of the retainer was nearly cut through.

Good luck!

__________________
'82 300SD - 361K mi - "Blue"

"Good judgement comes from experience. Experience comes from bad judgement."

listen, look, .........and duck.