John,
I would figure out what organization you plan on racing with first.
I would also recommend using dzus fastener (or other similar fasteners) so you can change the body work to match each organizations requirements. I race with Midwest Council and VSCDA around here and have no issues with the wider IMSA flares. When I went to an event run by SVRA they wanted to put me in with the prototypes or the Mustang/ Corvette group. Having only a 2.2 L six this was less than appealing. The wheel size will also come into play, VSCDA requires 15 x 7s for my car so using my 12” wide Gotti’s is just not an option. The IMSA race car I am now restoring will be capable of a quick fender and wheel change to get me into any vintage club.
The fenders from GT racing are called GT quarter panels but look similar to what came on your car…I guess you need to confirm what you would get from them.