In my opinion, there might be a market, but not necessarily in the area you are thinking. The "classic" muscle cars might not see much benefit from these swaps because the people who are drawn to them would rather be running high displacement, high HP motors.
You might be better off converting the less desirable cars from the 50s, 60s and 70s. Quite a few hipsters/scensters want to be different and driving something that matches their fedoras, cardigans, and 35mm cameras would fit the bill. They are only interested in image anyway, so they probably wouldn't even be bothered with what's under the hood. Then again, if they could brag to their friends about a 40 mpg "classic," that would be even more of an incentive.
Also, because of different safety regulations, many of the cars from that era are remarkably light sans engine. You could probably get away with swapping in ~ 2.0L diesels (with or without turbos) and be fine.
Anyway, I think your idea has legs. Never underestimate a person's willingness to pay to be different. What was it Walt Disney said about the first idea he would work on was the one that everyone told him wouldn't work?
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