I think it depends on how much you want to tweak it. Keeping the stock engine well tuned up is half the battle. A slippage of a degree of timing or a few degrees of dwell (point gap) can result in several MPG lost. I would start with a recurve of the stock distributor and electronic ignition to optimize things. A good hot spark will use all the fuel in the cylinders and make the engine start easier.
Second, one barrel carburators are tradeoffs between milage and performance. The Rochester is especially poor. A well tuned Carter will help, or even a more modern 2 barrel carb or fuel injection. Higher gears will really help, too. The 235 is a fuel waster when spun at higher RPM's. There is a limit to this, though. When the gear ratio gets to the point that the engine is lugged while cruising, the vacuum the carburator sees drops quite a bit, which causes the power enrichment circuit to operate in the carb and the vacuum advance to drop back, even on flat ground, which will definately keep the gas milage low. This can be even worse with a performance cam. An adjustment to the vacuum can and lowering the spring tension on the power valve will help.
I would try 3.55 gears with an overdrive transmission of some kind (T-5 or Saginaw 3 spd OD, or something) or 3.07/3.08 gears with a 4 speed direct type box, like a 2 or 3 groove Saginaw. Tires should also be a consideration. Tall, narrow profile (skinny) tires will get you better gas milage because of lower rolling resistance. They should afford plenty of traction. You don't need 70 or 60 series fat tires for traction with a 235.
Another enemy is compression ratio. The stock 235 head is pretty low compression for modern gasoline. Raising the compression a few points by shaving the head while doing nothing else should help fuel economy and torque.
Finally, the engine needs to breathe well. The air cleaner should not provide a significant restriction to flow as it will act just like a choke and cause a richer mixture. The exhaust should flow well, a low restriction muffler and cast iron headers would be a good addition.
A PCV system would be a good idea to keep the crankcase clean. Whether or not it will help with fuel economy is up to debate, but it will definately prolong the life of your oil and keep the air cleaner.
Be prepared to do a lot of tuning to get good fuel economy. My truck ('50 with a 261 and 4.11 gears) weighs about what your suburban does, and I have gotten anywhere from 11 to 19 mpg depending on the state of tune it has been in and driving conditions. I suspect you should be able to do 15-20mpg on a regular basis if you tune things well. All the mods you do will also increase your top speed, but resist the urge to drive faster if you want fuel economy.
I have a 3.07 rearend and a 4 speed saginaw that I am going to put in, and plans for a head shave and mild cam for the 261.
