I've done the swap in earlier trucks. My 59 had a sm420, so for all intents and purposes, it is the same swap.
YOU WILL WANT TO USE THE 1994-1997 DODGE 2WD VERSION OF THE NV4500. This version does not have the parking brake. The dodge NV4500 has a 1 inch longer pilot shaft than the gm version. This allows you to use a 1 inch adapter between the bellhousing and transmission. By using the adapter, most of your clutch linkage can be used.
You don't mention what engine you have (I'll assume a SBC). The adapter plate Advance Adapters sells is for a truck that was originally equipped with a sm465, not a sm420. The difference is the bearing retainer hole size. The sm465 bellhousings were indexed to 5.125 inches, while the sm420 bellhousings are indexed to 4.686 inches. The adapter plate is indexed to 5.125 inches. You have a few options... bore your bellhousing out to 5.125 inches, use a 1968 and later bellhousing that is already 5.125 inches, turn the indexing lip on the adapter plate down on a lathe, or make your own adapter.
As for the parking brake, you are on your own. I'm sure Grigg has a few ideas for you. There are companies that will swap tailhousings around on the NV4500, so you could have a dodge front and a gm rear and get the brake. You will have to convert the electronic speedometer signal to mechanical with an aftermarket device.
