Who would be interested in buying a 5.7 Hemi A Body car?
Pricing highly modified vehicle ike this for the market is, I think, alot like pricing a vacation home - each is different and unique and very subject to whim. An awful lot has to do with how it's done - who it will appeal to; much like the way the factories do market research on new models and carefully gauge responses to proposed new offerings at new car shows. In doing so they keep an eye on what colors are hot, and what options people want on a year by year basis.
This is different than doing a ground up restor, where there is a more or less known audience and a book value for these - and they can get appraised because of that.
I do sense that there is a market for this kind of thing, but there are so many ways it can be done and so many ways it can be messed up that I'd be very careful to guess a price point (hate that phrase, but that's what they call it).
Personally, it's the kind of thing that, done right, I would like; for the same reasons as already given ... new tough engine, lighter weight, better mpg, good power. But I would be watching closely for engineering quality ... header clearance, steering mechanicals clearance, engine mounts, transmission apparatus, ... all the custom stuff that can really become a nightmare if not done correctly - in short I'd be a wary buyer.
If I were to look for one, I'd only want it if it were EFI ... if it's carb'd I'd just as soon keep to the old LA.