The italian geasets are available........for Muncies. And no, your SM 420 isn't a Muncie. It's a Saginaw. Lots of terms have been thrown around for years by those who don't know any better, and those that should. Believe me, if a simple overdrive fix for an SM420 is possible, it would have been done by now.
You could buy an expensive overdrive like a gear vendors. or a less expensive overdrive like a Ranger overdrive. Or maybe you could luck out and find an old Spicer high/low box (2021)
But the path of least resistance is most likely the T-5, NV4500, A833 avenue. Plenty of info on the interweb and good availability of units.
