The resale value of a car modded in that fashion would plummet. You have a car that is worth, say $7,500 now. Add the engine you are talking about for another $15,000, and the market value would be $5,000 for this "upgrade". However, if you rebuild your engine (even though it may not be original to the car- or better still, find and rebuild the "correct" engine), you will increase its value to over $10,000. The decrease in market value to the car will more than offset any fuel savings.
Go pose this over in the "vintage" section of this forum and see what the reaction is.
Rick