Flywheels and flexplates are usually referred to by the number of teeth they have on the ring gear: 153 or 168. The diameters are approximately 12 and 13 inches, respectively. It sounds like you have a I6/SM-420 combo with a 168 tooth flyfheel and matching clutch...and a 350 with a 153 tooth flywheel...and a matching starter.
So something's gonna have to change. If you want to keep the larger flywheel (and you're gonna use the I6 bellhousing), then all you'd need to do is get a starter for a 168 tooth flywheel.
If you want to do it the other way around (keep the 153 tooth flywheel and starter), you'd need to get a smaller clutch and pressure plate.
I went thru this when converting a Chevy II from automatic to 4-speed. It was a pita, but eventually you can get all the right parts in there at the same time and it'll work.
