The reason for Brian's comments about a clevis pin is that if you have a small diameter pin vs. much bigger whole in the chainplate (or whatever) -- then the loads on the pin aren't distributed evenly along the pin; rather they are concentrated on the point of contact. That way the pin takes stresses in an unnatural way -- could become distorted, suffer fatigue failure sooner, etc. etc.

The CLEVIS pin, as you point-out does take load.

In the case of a chainplate, the loads are only axial and (for our purposes) only in one direction. Bolts work in shear in a perfect world, and otherwise would experience some bending, to the extent that there was some play between the backer plate, FRP inbetween, etc.

We are all handicaped by not knowing if i) the holes in the back-up plate put the chainplate evening at the "top" edge of the 1/2" bolts vs. 9/16' holes. (If one or two of them are at the top, they'll take all the load before the others. After the system "gives' or moves a bit, then the others would yied. In a perfect world they'd all be loaded pretty evenly and nothing would move.

I think, as long a Dan y is correct about the plate from Garhauer EXACTLY maching, and the SS bolts he took out looking pretty undamaged, as long as he gets exactly the same quality SS bolts and reinstalls the assemby, they should be good to go. If the strength of the bolts isn't the same, or the bolts were somehow deformed, etc. -- then the result might not be the same. The rig in that Gulfstar has seen a lot of years and miles with no reported problems until now?