Unless the box has fallen down it is a major PITA to drop it down, drill out existing pop rivets and resecure the bracket to the box. The rear of the electric box inside is not accessible because of the masonite board that has been populated with terminals, relays, resistors, diodes and circuit breakers.

Getting the box down and on its face so the pop rivets are accessible involves not only removing the screws holding the brackets to the bus bulkhead, but also cutting a number of nylon ties, removing some small coolant lines and removing the rear oil pressure gauge and sending unit. Without a doubt rivnuts would work but I doubt if many here want to go to the effort.

What the goal is here is to reinforce all boxes that have not had a failure of the pop rivets so the total reliance on the pop rivets is supplemented with a second bracket that can carry some of the load and possibly prevent the pop rivets from failing.