I applaud his decision to use a sedan to move the 5 big folks instead of the more usual "need" to get a Subdivision or Explosion.
I also understand that since this car practically fell in your lap, that's what you're going to use. Just to mention, though, if you come across a GM full-size fwd with the 3.8 (think Le Sabre, Bonneville, etc.) from the late '80's on, they should give an honest 30 mpg hwy properly driven, stock. For some reason that 3.8 is a decent fe package in those larger cars, equalling or besting the fuel efficiency of the same model with a smaller engine! I wonder how they would perform with a 5-speed stick and removal of all those power-robbing accessories?
IMHO, the four-valve head's main strength lies in high rpm applications, OR, if equipped with a variable valve control system of some sort, low and high rpm applications. Since it is unlikely you'll transplant a variable valve control system (and I don't know if it was ever offered on that engine series???) AND since low rpm operations are our fe friend, my instinct is to stay with the 2-valve head. You may want to research fe ratings for different versions of that engine in as consistant a drivetrain package as possible to see what is really going on out there.
If the 6 speed offers a taller OD than the 4 speed it MAY help. If cruise rpm is too high for best fe, higher rear end gears and/or taller tires could help too without being as big an effort or expense.
Even with these mods I have my doubts about getting 30. It's just such a tall order for a V8 especially in a full-size package. Seems the odds get much better with a 6 cyl.
Old EPA 23/33/27
New EPA 21/30/24
Well a gear swap may not work. Whats in there now? It is a big car, and those Mod motors like some rev's so if you go to too tall a ratio, you could drop the motor out of its power band, and make the millage worse.