When my buddy and I bought the '68 Z, it had a M20 '70 fine spline/400 output 661 cased Muncie on board, with the Hurst universal (plate/U-bolt) tailshaft mounted shifter base, and a Competition Plus console shifter mounted to it (car is also an original U17 option). Also on board, a Lakewood steel bellhousing, aftermarket 11" flywheel and a cut driveshaft.
Sure, it drove and shifted fine, but not original by a long shot. Since the car is a documented original, and came with an M21 originally, we started putting it back together from the flywheel out. Took some time, but the originality is worth it in my opinion. Down side - it's an original Camaro '68 M21, but not to this car (I'll post the VIN one of these days), it still uses a Hurst shifter (with a Muncie handle), but it functions like it should and still looks basically stock. We sold the '70 Muncie, bellhousing, flywheel and used the proceeds to buy the Muncie, 403 bellhousing, driveshaft and even a '68 dated 021 flywheel and the proper starter. Actually came out pretty close to even - just have to dig -
Regards,
Steve