what are my realistic manual transmission swap options?

I would go go with the 4 speed or 4 speed OD since all of the factory humps, shifters, linkage, pedals, bellhousings and clutches are available. I'd go A body(Duster Dart) or F body (Aspen Volare) since it is simpler. The only difference between /6 and small block would then be bellhousing, flywheel and clutch. A parts car would be the best since you could get almost everything you need in one package. I have no experience with the others.