Look up information on swapping in a new-generation Chrysler Hemi, either a 5.7 or 6.1 liter. There are few classic Mopars running around with this swap, and you can get reasonable fuel economy and performance both.
Then again, why are you that concerned about fuel economy in Pro-Street car, anyway? Surely it's not daily transportation.
If I were in your situation, I'd just just save up $30,000, open up a Mopar Performance catalogue, then order part #5153312....The 540ci aluminum Hemi crate engine. Save the vintage Challenger for fun, then use the Challenger V6 for commuting.
