Yeah, that's it. You could drill the rivets and get a look at what's actually behind the plate. Mine has small bolts and nuts that the previous owner fitted. You may be able to use a standard J-bolt and then bolt the plate back on to re-captivate it.
I can tell you that the trays in my BJ74 have been into at some point because mine has little bolts and nuts instead of rivets. The tray I photographed is a vehicle in the museum.
No point as far as I am concerned. I already have spaced wheels. I'd like to forego spacers. In addition I've never seen 30mm (I think you meant 30 and not 300) spacers.
On the other hand, that would work nicely on an 80.
Those numbers are essentially correct. It will depend on the given wheel's actual flange width. It took me a very long time to wrap my head around backspace and offset.
Wheel width is stated as "bead-to-bead" meaning this is how far apart the tire bead contact surfaces are. The wheel outer...