What transmission and rear end does yours have? Mine is an M46 and 3.31 axle, which I think is the tallest gearing possible with readily available parts. If any taller gears are possible, the car feels as if it would have no trouble pulling them.

Other things to look into are a free flow exhaust (mine has turbo parts on a non turbo car) tires as narrow as possible, pumped up hard, removing roof rack if equipped (my next step) and thinner oil (I've been running 15W40 in mine, next oil change will be 10W30)

The 90-92 front end with composite headlights is more aerodynamic, so for ultimate 740 mileage you would want to start with one of those, preferrably a "no letter" car, which has no sunroof, saving about 50 lbs.

From 85 to mid 88 the 740 had the "low friction" engine, with smaller crankshaft bearings. This may get better mileage than the late ones with the heavier (and more durable) crank.

The radio antenna creates a surprising amount of drag. It might be worth looking at alternative types and locations, especially on a wagon.