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.
