I thought I heard sometime ago that there WAS a problem on the Shelby alloy block that would not allow for an OEM Ford starter? Which I suspect, like the Kirkhams, is so seldom used anyway this issue may have dropped out of view? The problem may have been corrected on that block now, the early ones did have a couple of other issues as well (like
oil return hole for the rear main seal).
Recently the starter drive went out on my "Mini Torque" starter and I can't find a replacement drive, SO, I put the OEM starter back in. It works mostly OK, unless the engine is hot, shut down, and a restart is fairly soon. In that case the OEM starter just barely turns it over and draws a LOT of current doing it!