looking under the hood of a typical 70's GM car as well as cars like my father in law's 98 Olds 88, you can still see plenty of tarmac around all sides of the 90 degree spacehog engine. FAR TOO MUCH wasted length under there. example, why should the radiator have 6 inches of clear air in back of it of which some of could be returned to the passenger compartment? Sit in the front seat of an 88, and its cramped, the dash is low and way too far back into the wheelbase of the car. as a result, there's no front leg room and no rear leg room.
Look at a mid 80's caprice. ENORMOUSLY long hood and trunk. what's left in the middle for leg room?
A good recipe for a full sizer would be about 116 to 118 inch wheelbase and an overall length in the mid 200's. there should be a minimal front overhang over the front and a longer rear overhang is ok if it benefits the trunk. I like a low cowl and dropaway hood (like my 99 taurus or like an early dodge stratus) for open airy cabin and visibility. high enough roof for higher more natural seating position and more headroom. fairly low side windows. a dashboard that is less upright and claustrophobic. stretch the roof back far, again for cabin spaciousness. long/wide doors that have almost 90 degree hinges.
then, if the car is kept around 3500-3600 pounds in FWD form (FWD is most space efficient) it should get great mpg, handle well for a big car, go like stink and have an assload of room.
Dodge regressed moving away from the last Intrepid blueprint. the intrepid had a bunch of room, but it did suffer for seating that was too low and a roof that was far too low.