Here's some better pictures showing the differences in the rear panel and the quarter.
Here's a stock '74, should look a lot like yours. One thing to notice is that the rear bumper is actually wider than the tail of the car, by quite a bit. So the plastic filler actually angles out quite a bit.
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Here's a stock '73, although the '70-72 cars should look pretty much the same except for the tail and marker lights. You can see the large flange that sticks out from quarter at the recess for the bumper.
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So, if you mount a '70-73 bumper on a '74+ car, you get a gap like this one
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Notice though that in the picture above the bumper is pretty much the same width as the tail of the car, the bumper doesn't stick out past the quarter panels like the '74 bumper does, so the plastic filler is the wrong shape. All you need is a flat piece like the early cars had. I looked at a bunch of pictures of the original cars, then made a template and some filler pieces. I made the filler piece out of two parts and welded them together, just like the factory 70-73 cars so it has the same look. Then I welded it to the flat end of the quarter panel and filled it
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And the "finished product" with the bumper in place looks like this, pretty similar to the 70-73 cars. Obviously I'll have a little more bodywork to do here when I paint the car.
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Also, I filled in the holes in the rear panel for the shock mounts. I just made a couple of patches and welded them in. Then, I made a copy of the reinforcing flange on the frame rail. The other side of the rail has this flange for the bumper mount, so I made a copy and added it behind the patch.
Here's the other side of the frame rail, the arrow shows where the flange is and how the hole is actually still in the flange to locate the 70-73 style bumper brackets.
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The other side of the rail, showing the back of the patch I welded in and the mounting hole for the 70-73 brackets
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Here's the flange I made up
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And the whole thing welded in place. Now the 70-73 standard bumper bracket just bolts in place like stock.
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