Moderately difficult but usually out there if you look (try Moparts classifieds), and when located, usually quite expensive. There are a lot of small parts worth a LOT of money so a truly complete ready to drop in with all of the original detail parts (66-71 Hemi) can be far more than most imagine.
BE CAREFUL OF JUNK! There are A LOT of damaged patched together Hemi blocks and heads out there! Many of which have been used to build an engine out of junk parts for hopes of making a quick buck off of an uneducated buyer.