[JRTJH]

This is not criticism, anything you do to improve the attachment is better than what was used and failed. That said, from an engineering perspective, a grade 8 bolt must have a specific number of threads engaged "in the nut" to meet strength requirements. If you "tapped threads" in a 1/8" mild steel A-frame member, you may not have sufficient strength to hold the "grade 8 bolt" without failure of the threads or the ability to hold the bolt without loosening...

Typically, a 5/16" nut is at least 1/4" thick and most grade 8 nuts are at least 50% thicker than that minimum. There simply isn't enough "thickness in the A-frame channel to hold a large grade 8 bolt securely (without stripping out).

Unless you've installed nuts "on the underside of the grade 8 bolt" (I'd suggest lock washers as well) you may be "working with a false sense of security and a pending failure".

As I said, don't consider this as criticism, what you did has improved the tray security over what the factory did, but unless you've got nuts and lock washers under the tray/A-frame assembly, the threads in that mild steel A-frame probably won't hold a 1/4" or 5/16" grade 8 bolt "indefinitely"...….