NV4500 vs SM456 vs SM420

[shawn66mcl]

I'm sure this has been brought up one way or another, but my question revolves around my transfer case. I have a 67 K20 with a Rockwell T221 and a good condition SM420 along with a spare SM420 transmission that needs new syncros on 2nd gear. My issue is my T-case is pretty much shot due to no parts per say available for a rebuild. I want to swap to an NP205, but I'm not finding much in the way of adapters for an SM420 to an NP205. Ideally this would be the cheapest solution but if I can't find a way to do that, I'm considering an NV4500 so I have overdrive. I know the SM456 is a little cheaper than an NV4500, but I plan on actually doing long trips on the highway and what not and thinking the of might come in handy. I don't need the super low granny gear and for a difference of about $600, I think it might be worth it for the newer trans. So if anybody has any suggestions on an adapter to the SM420, the best place to purchase an NP205 or also the best place to purchase an NV4500 would be greatly appreciated. I'm pretty handy and could rebuild any of them with a manual so I know junkyards, but I'm not opposed to buying a refurbished unit. If there's something else you guys can think of I'm missing or could use would be greatly appreciated.