55-2nd series fuel tank bolts -what's correct?

[ndamerau]

I took the fuel tank out of my 55-2nd series 10 years ago and I am ready to reinstall it (get out of my way, I am moving fast). The bolts that I removed were obviously not original; they had loose nuts under the cab. The holes have nuts welded to the cab underneath, but the holes are stripped/drilled smooth, so I can't tell if the bolts are supposed to be 3/8-inch or 5/16-inch.

Anyone out there know or have a picture of what is supposed to hold the straps down? This truck is almost all original, so I am trying to keep it that way.