Where did the converter come from? I believe that all C4 conv are either 10.5" B.C, (case fill, 157 tooth flywheel) or 11 7/16" (pan fill w/ 164t flywheel). I have a 460 C6 flywheel, and it also has the 11 7/16" pattern.
hey, cheater, I had one of those too. It was an old 9 3/8" bolt circle, out of an old bracket car. Supposedly, this guy told me that it came factory in mustang II's but I dont remember MII's coming with c-6's. I think you might be able to have the flywheel drilled if you want to use it.
I dont think you can drill the holes in the existing flexplate. If memory serves me correctly, once you look at it, the dished part of the flexplate is right where the bolts would be if you drilled it. You might try JW performance. I found some weird stuff there, like turbo-350 on a small-block ford (??), etc. They really are good folks there, and should be able to point you in the right direction. Hope this helps.