What engine for my new Town Panel?

I believe you're right - if it had a V8 (whether poly or 361/383), it would have had a push button 727 in 64. For the conversion I believe I would try to find a parts truck with the Magnum motor and the 4WD 518. I'm not up on 4WD so don't know if the 518 uses the married (or could use) the divorced transfer case.