The SM420 has a different bolt pattern than the old sliding gear transmission. A clutch housing that fits a SM420 will fit your engine, but the engine mounts will be different. The fix we did 60 years ago was to replace the 1947 and earlier cross member with a 1948-1953 cross member. It has been many years since I saw a conversion done and I don't remember all the details. But a 48-53 clutch housing, cross member and as SM420 is doable. And my generation did it when we were teenagers in the 1950's so it wasn't all that complicated.