No harm to just put the TC on a foot back. If you can still tie it down to the truck and the TC floor is fine hanging off a little (or tailgate down?) then no issues. Dont load the TC with weight at the back as much as you used to of course.
I had a truck where I kept two GC batts in front of the wheel fenders. The batts were always with the truck. On the same truck in a newer model the fenders were different and of less room and this was not an option. I would not think it cost effective to custom make two tanks to fit there, but maybe it would be in your case.
Check with welding shops, $1,500 sound high for a couple hrs work welding and cutting...
You make a model out of cardboard and take it in to be replicated in metal will save lots of money.
Have you checked under the truck for more room?
Run the exhaust out in front of the rear tire and that opens up a lot of room for another fuel tank. You can buy a generic tank and bolt it in, similar how the stock tank is bolted in. Or get a junk yard tank and have the same 35 gallon capacity as your stock one, and keep TC in place.