[aerohead]

This is the issue. In order to achieve any significant highway improvement your looking at 'epic' transmogrification. The appearance of the Mark-IV would be only a shadow of it's former self.
The Lincoln was designed 'pre- Energy Crisis', and after October, 1973, engineers, energetically went to work, downsizing ( mass and frontal area ), powerplants, improving transmissions ( overdrive, electronic lock-up torque converters, more gears ), introducing higher pressure, low aspect ratio, low rolling resistance tires, electronic engine management ( fuel and ignition ), lower power consumption engine cooling fans ,Messier 12-VDC electric water pumps of low hydrodynamic drag design, softer piston rings, roller-cams / tappets, high-swirl combustion chambers, tuned exhaust, synthetic lubricants of lower viscosity ( engine, trans., differential ), taller gearing, front wheel drive, and modest aerodynamic tweaks, etc..
--------------------------------------------------------------------------------------
If you cannot lose your attachment to the car, the field is wide open for modification, however, your engine is of extremely low thermal efficiency, compared to contemporary powerplants, and with extensive modifications would still be at least 15% lower that of a typical modern car.
-------------------------------------------------------------------------------------
Vehicles of the Lincoln's weight now have 10-speed electronic transmissions.
Half the drag.
-------------------------------------------------------------------------------------
If you want the challenge, and you have a bottomless wallet and calendar, and you have nothing else to do with your life, then it would be a 'fun' project to see what you could pull off.
Like freebeard has said, anything can be improved, you just need to do the soul-searching, and figure out if it's worth the effort.
It's a $90,000 project, if you farmed it out. Without an engine.