[aerohead]

Is 40 mpg your highway mpg target,or for mixed driving?
Do you already have an mpg baseline for the car that we could be thinking about.
If I can get 36.5 mpg at a constant 65 mph in a 4,320-lb,midsized pickup truck,then with less frontal area,and less weight,I think the Crown Vic might see 40.But only on the highway.And not at 70 or 75 mph.
Modern Ford automatic transmissions are as mechanically efficient (92%) as a manual trans when in lockup.If you're turning in the neighborhood of 2,500 rpm or less at 65 mph,your present gearing is probably okay.
You'd be looking at rear-end body work which would mimic as close as you could,the 1985 Ford Probe-V concept car.