My post above post might be a little confusing.
Because you would simply be placing an adapter plate between the bellhousing and the transmission, Most of the original clutch components can be reused. This makes ordering parts much easier in the future as you don't have to have a speacial book with part numbers. Just look up the clutch by the year and make of your truck.
I have posted some pictures of the sm420 and the Nv4500 side by side on the oldgmctrucks forum.
The basiscs of this swap is that the dodge NV4500 uses a pilot shaft that is one inch longer than the gm pilot shaft. This allows you to use a 1 inch adapter plate between the bellhousing and transmission. The adapter plate simply bolts up to the bellhousing, and the tran smission simply bolts up to the adapter plate.
Advance adapters could hook you up with a crossover pilot bearing and a crossover throwout bearing that will work with your clutch fork. I turned the snout of the bearing retainer down to match the snout of the sm420 bearing retainer. This extra machining allowed me to use the original throwout bearing. (It has been several years, so don't quote me on these numbers, but I believe the snout of the dodge NV4500 bearing retainer is 1.43 inches in diameter, while the snout of the sm420 is 1.383 inches. That means about 60 thousandths need to be cut off the dodge bearing retainer for the gm throwout bearing to slide over it.
