The easiest and cheapest solution would probably be any of a variety of 3 speed (truck or car) or 4 speed (passenger car) transmissions. The newer 5 speed overdrive transmissions can also be fitted, but the cost and effort goes up. As purchased my truck (1960 GMC) originally had an SM420 and now carries a Richmond Super T10 4 speed. It’s certainly a lot more fun than the granny box was.
You might want to take a look at the details in this link, particularly regarding the diameter of the bearing retainer. If you have an SM420 instead of an SM465, I don’t think this is an issue.
http://www.novak-adapt.com/knowledge/sm465.htm
And welcome to the forum!