It weighs about 125 lbs. so I wanted it to be secured to the truck, not the wood floor.Why not install rivnuts in the sub box and bolt from underneath? Would be a much cleaner install
It's hard to find clean trucks these days. But they can be found. That picture with the frt fender off looks better than some I've seen. I had one that I thought was way clean but that support apron was rusted thru. My kid rolled one and that rear body mount you have a pic of broke free.
You need to go find a new body. Better still you should look at those wildcat chassis.