The SM 420s weren't made at the Muncie plant, nor were they made at Saginaw. Those plants produced mainly 3 speeds (car and truck) and passenger car 4 speeds. The SM 420 and 465 were actually made at the Toledo plant.
BTW, the "SM" designation on GM manual transmissions stands for "Syncro-Mesh", which was developed by GM in the mid-'30s.
Also, "New Venture Gear" is the GM side of New Process Gear, with the older name being the Chrysler side.
One more thing, unless you can dig up all of the components to do the 3 speed swap, I would suggest using the T-5 as well.
