I forgot how fun this bolt was. I had this same problem way back when I put a body lift on TRS-1.
You need to get a saw blade between the frame and that clip and saw that bastard off. It's going to take forever and you're going to be miserable and hate it.
Or you could make your life easier, say the hell with it, slide the bed back, lower the tank, and use a grinder (preferably with a cutoff wheel) and cut if off. Or a Sawzall or another cutoff tool. If you can't get in between the bracket and the frame you should be able to cut it flush from the bottom and then work it out of there. If you can't find a new clip, re-use one of the good ones here, and just use a nut, lock washer, and washer somewhere near the rear that's easy to get to with a wrench.
This would also be a good time to wire wheel the inside of the frame. This area of the frame traps a lot of dirt and crap that rusts it out. Looks like you have a lot of rust here.