All rumbles used the same part. The original curved inner panel would have a square hole for the bracket.
You should not need to buy the repro it is a fairly common part out in the wild. A good curved inner panel is a different story.
The repro curved panel is not right in many directions. You will have to adjust the position so the bracket mounts tight to the lower rear panel. If not then you will distort the panels when you bolt it together. I needed to beat a bulge into my repro panel to get it to the right position.
https://www.cabriolet.piklefactory.com/Rumblelid.htm