A t5 would likely be an easier swap, as would the nv4500. The t5 should be plenty strong to go behind a 250. The nv3500 has that integral bellhousing to complicate things. In my opinion it would be easier to get a 54-55 first series bellhousing crossmember, 55-67 v8 truck bellhousing and mechanical clutch linkage, and a T5 to go behind it. You would likely need a new clutch disc to match the T5.
With the transmission you have, you would have to remove the bellhousing crossmember, fabricate a new transmission support, figure out how to run the hydrallic clutch, and eventually convert the electronic speedo signal to work with the mechanical speedo. Not all that much work or figuring, but more work and figuring than I would like to do.
I believe the NV4500 is probably too much transmission for you, but it is interesting to note the 1994-1997 Dodge 2WD version can be made to work with the mechanical speedo very easily. I have the NV4500 in my 1959 GMC 1/2 ton and love it.
