yes and no to your questions.
Can You run the sm420 bellhousing... yes, but you need a 1 inch adapter plate, and the dodge nv4500, or at least the dodge pilot shaft and bearing retainer collar. There will be some extra machining involved on the Advance Adapter plate as the one they have for a gm bellhousing is indexed for the later 5.125 inch bore bellhousings. The Sm420 bellhousing is indexed to 4.686 inches. The indexing lip on the adapter plate can be cut down to fit, or if the bellhousing is out of the truck it can be bored out to 5.125.
The Dodge standard duty NV4500 and the gm NV4500 use the same spline count which is 10 spline on 1 1/8 inch shaft, which should be the same as in your truck now. There is a HD dodge version that went behind the diesels and the v10s that had a 10 spline 1 1/4 shaft. This transmission can be used if you swap out the bearing retainer and the pilot shaft for the regular duty dodge NV4500.
There are companies like "High Impact" that might be able to design a kit for your install. http://www.high-impact.net/transmission_and_gear/nv4500.htm
The 1994 -1997 Dodge 2wd NV4500 is the most popular for an early chevy swap for a few good reasons.
1. The mechanical speedometer will work with this incarnation of the transmission.
2. Most of the stock clutch components can be used. I spun down the collar on the bearing retainer on a lathe so it would accept the stock gm throwout bearing. There is a crossover throwout bearing for the swap that does not require machining the collar. The Pilot bearing will need to be ordered speacial or fabricated. The gm transmissions have a pilot tip that is .590 inches. The Dodge pilot tip is .750 inches.
3. The dodge transmission will work flawlessly with a 1 inch adapter plate because the pilot shaft is 1 inch longer than a chevy pilot shaft. The adapter plate is prefered when the bellhousing mounts to a crossmember. You will not have to change motor mount setups this way, or clutch linkages.
Can a gm NV4500 work in your truck? probably, but you will have to figure out things like new motor mounts, speedometer hookup and new clutch linkages.
For the record, I have not done this swap on a 60s vintage chevy. I have done it on a 1959 gmc that had an sm420, and a 1940 gmc with a 6 cylinder.