Mancini sells a 426 cid, single carb, 465 horse crate engine for $14,305 so you're getting a $2k discount on a used set up. How used seems to be the question.
Clarify with him where the Hemi came from and what the mileage may be. I think everyone agrees that vintage units can be somewhat suspect. However, some of the early crate offerings had some questionable assemblies in them as well, so they aren't a slam dunk that they are good. However, I think the odds of a later model version having more meat to rebuild on are probably better than the same possibility on earlier engines. That means if it isn't good, the rebuild won't cost as much as welding and sleeving potential on an earlier block.
Of course, it could be just a decent used engine that is a fair deal.