Horsepower with mileage options

That sounds like fun, but you also stated your son was still in school, if he is truly looking for MPG than the money spent on modifications, would be better spent on any 15 year old 4cyl car, pick a clean one somebody local has for sale.
It will be cheaper and provide better mileage than an old school car.
Sorry to be a party pooper, but it is the truth.
So true. We have a shop truck with 580 HP and a 4 cyl Dakota shop truck. The 580 HP 1974 1/2 ton hasn't moved for 11 months and the Dakota is used every day. My vehicle is a 2000 Dodge Stratus. Bought it for $200 with a broken timing belt and 13 bent valves. For another $150 it now has a new belt and 16 new valves. About 26 mpg around town and 37 on the hwy.