You have a 68 HEMI transmission. The C97621 is the large diameter 4.805"dia, 308 bearing with 4.16" hole pattern. The bearing and hole pattern are on your transmission so you can't change that without swapping case. Your trying to stick this in a Bell House that need a 4.354 diameter retainer.

So messing with a BH is problematic. Your alternative is to get the over drive retainer that has 5.125 dia, 308 bearing and 4.16 hole pattern. Turn it down to 4.354. This would also be pretty close to cutting into the holes of the retainer. This is much easier to do. BUT since your a 18sp, they will also have to put a sleeve on the tube the throwout bearing rides on and bore the diameter out. Lot of work, but I think better then working on BH. Frankly I would look for the 68 BH.

I have turned a OD retainer for a 308 23 spline to the large hole 4.805. That was easy and worked. You really may be stuck with changing Case or BH.

Is there enough meat on an 18 spline retainer to turn it down to the size your bellhousing is?

As you said, your trans isn't the "correct" one anyway so if you can source a compatible bellhousing, that's probably the easy button solution provided it's compatible with your clutch and flywheel.

Most of the car show "experts" will be too busy yammering on about their uncle's factory 383 hemi 62 Valiant to notice anyway.

Kevin