believe I had this problem partially before. I would zip cut the nut off from the head light bar.
To get to the rest of it still attached inside the headlight pot, drill out the rivets and separate the bottom mounting plate from head light pot.
Those carriage bolts are strictly automotive and diff square head design than the ones you see at home depot . They are smaller or larger
in the head. You should be able to use a larger square head design and file down the square and also belt sander on the edges of the head to reduce size so fits down snug. If you cannot find the automotive ones
.
I would use a button head allen head instead in its place and pre-tighten it.
I would also use 10/32 button head allen heads to replace the rivets with nylocks.
I have collected a pair of original 34-35 chev standard head lights to replace the Guide sealed beams I installed in early 70s. Might have to take them apart as I described here.
Finally got the password to work again, so am back
mike lynch
mike lynch