I could be wrong but I believe the time and mission side adapter is two pieces that bolt together. The transfer case side has the bearing features in it. The transmission side is what I was thinking he could modify.Yeah, you have a aftermarket tailhousing on the NV4500. a stock one from a Dodge will be a simple 6 bolt round pattern and much easier to adapt to. Additionally, The GM tailhousings are about 2" longer the Dodge version tailhousings so MAYBE if you use a DODGE tailhousing with a output shaft intended for a GM tailhousing it will stick way out past and give you the room for your adapter.
I think making a new adapter that bolts to both the Marlin SM465 adapter and to the NV4500 tailhousing and leaving both unaltered would be the way to go if you can. The Marlin adapter to the crawlbox seals it and holds bearings, right? I would not want to get into re-engineering that if you don't have to
As for the dodge vs chevy tail I don't know how those work but I assume they use the output shaft without a coupling and the typically pass directly into the transfer case.
In his instance the output needs to be 10 spline. I don't know if they have a 10 spline output or not. They might just use a spud shaft or a coupling. One that is female with the 4500 pattern on one side and the other pattern on the opposite side.
Hope that makes sense,
Mike