The advantage will be in the shifting. Other than that the original transmission will take anything you throw at it.
If you want to go with the sm420 then use a bell housing from a 47-52. Make sure the motor mounts are horizontal and not at an angle.
Send it over to Buffalo
http://www.inliners.org/buffalo/index.htm, they will weld on mount points for the clutch and brake.
Re- use your original fly wheel, pressure plate, starter, etc.
You probably know the SM420 is longer that the original so you will need to shorten your driveshaft.
Bolt it all on and enjoy!
