There was a purple 70 challenger with a single turbo big block mopar making 800 hp and he was getting 17-18mpg highway. Key was low duration ,overlap turbo cam and was tuned lean at cruise speeds with efi and overdrive trans, not cheap though...
Sounds like a cool project.
I have an Ultradyne custom grind blower cam that has zero miles on it .Pulled it out when I went to turbo.If your interested make you a great deal.
Here is a clip of it before I put in the turbo cam
http://www.youtube.com/watch?v=28qv2PiiT7I