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Some have a jam nut on the back. Some are just very tight or rusted. PB blaster may help. I use a tool that when you hit it with a hammer the screwdriver end spins to take out the screw and it come with a large phillips bit. Harbor Frieght should have it in ametal box about 6"X3"X1" thick.
That tool is called an Impact driver and usually comes in 3/8" drive and has several bits. Another trick that may work is to use the biggest philips head screwdriver that will fit the screw, get a death grip on it and turn it counter clockwise and use a hammer to hit the end of the handle as you turn. If it won't turn, spray on more PB then hit it harder.
I have just welded them up and added metal to fix cracks. The most important step before installing the covers is to reassemble the seats with the bumpers and make sure they line up as most bottoms are bent up.
I went ahead and installed the foam and cover on the passenger seat back last night. I just couldn't wait to get started. It looks great! I hear the bottoms are tougher to do, I guess I'll find out after I get the frames in order.