For MPG at higher speeds like 55 mph, aero mods will be a huge benefit. Since you said you have no limitations, right off the bat I will suggest a boat tail, and since you already need a front bumper, I recommend a nose cone. Also, grill block, belly pan, remove mirrors (if legal in your area), wheel spats... The sky is the limit.
Take a look at WeatherSpotter's Matrix, Basjoos' Aerocivic, and 3wheeler's Insight.
Take as much from those cars as you can.
3wheeler's thread in particular may be especially helpful since it's also an insight, and he did an incredible job with his belly pan and boattail.