[vectorit]

Any direction you take going forward will cost a good chunk of money I am sure.

Just my take on this subject in short.

O/D transmissions will always help
Gearing in the axle.
Add on O/D - Gear vendors device
Fuel injection is more efficient
Better cylinder heads - Those old double hump heads tend to have big intake valves. Aside from removing them to verify, you can look up the casting number found inside in between the rocker arms.
Ignition timing - Not sure why your vac advance is not there?
Tire pressure
Clutch fan
belt driven accessories like AC can add stress
Brakes, make sure they are adjusted so that none are dragging.

All in all, the Suburban is a big heavy barn of a truck moving down the road. Even the new Suburbans with all the latest technology gets pretty low MPG.