Questions regarding repair of rear cab mounts on 1991 Chevy K1500 Pickup

Hi everyone,
I have a 1991 Chevy K1500 4WD short-bed pickup. The rear cab mounting brackets have rusted through and the rubber cushons or mount pads are
falling through the steel brackets, which are welded to the frame. As such, the cab is sinking down towards the frame, and the cab is sitting on the support bracket on the drivers side.
The front cab mounts and steel brackets look fine, but the rear ones are rusted through. What really concerns me is that on the drivers side, the cab almost appears to be putting force on the top of the gas tank, and considering the age of the vehicle and the fuel lines and tank, I'm worried this may cause a fuel leak if the tank moves.
I have purchased the rubber mounts, lower retainer, and bolts from the dealer, but the dealer parts department said that the actual steel bracket is not available since it is part of the frame.
The dealer body shop said they were not interested in doing the job unless they could do it right, which means cutting the old brackets off, welding new ones on, and then installing new cab mounts, which means spending at least $1,000.00. Since the parts department said the steel brackets were not available, I'm not sure where the dealer body shop was going to get them. The dealer said that in order to even weld a metal plate on top of the existing brackets to repair the rust hole, the cab and possibly the bed would probably have to come off, which I guess is where much of the expense is coming from, but I'm not sure why they have to remove the cab or bed to weld a plate on the existing bracket.
Does anyone know where I could get an aftermarket replacement bracket ? I know they sell aftermarket panels, but I'm not sure about cab mounting brackets. I may be able to bolt it to the frame instead of welding it.
The steel bracket has a horizontal flat surface and then a part that angles down from the flat. I want to put a steel plate on top of the existing bracket, I won't weld it as the weight of the cab & the center of the rubber cushon will hold it there. On the bottom side of the bracket, I want to make a 3/16" thick steel liner that is shaped like the bracket, so that I can bolt the liner to the angeled portion of the existing bracket. If I can find an aftermarket bracket , I can use it as a template to make the liner. It' looks easier to bolt on a liner, rather than take the old bracket out and replace it with a new one (if I can even find a new one).
I might also take it to an independent body shop and see what they say, $1,000.00 seems a little pricey for this repair to me. The rest of the truck is in great shape, I'm not sure why the metal rusted through on the brackets, in the same place on both the drivers and passengers side.
I would appreciate any advice or suggestions anyone has.
Thanks John
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