Crunching your numbers and comparing them with my actual driving performance numbers I'd be willing to say if the majority of your cruising was going to be from 55-70mph you're about as close as you can get with your current combination. It would have you running 2044 rpms at 65 mph and 2201 at 70 and based on my own experience these would be good solid performance numbers as far as gearing.
The amount of torque you could pick up by adding a spacer would be minimal, about like adding an additional carb. Adding dual carbs is not going to be a "kick-in-the-pants" type improvement, but you're going to get better fuel distribution and actually improve your overall fuel mileage if you drive sensibly. A lot depends on your dual carb selection and getting a carb that can be fine tuned and will hold that tune.
As for the aero on your "Shoebox" I wouldn't loose any sleep over trying to make improvements. With suspension upgrades, good rubber, sway bars front and back and decent disc brakes the AD will click along at 85mph even with the straight axle. Granted, it's not something you'd do comfortably for miles at end, but the 235 will easily reach that speed with the upgraded gearing. Of course you're going to be reaching the top of the torque range and your fuel economy will suffer.
And suffer it will with the E10. In addition to a 5-25% decrease in fuel mileage, the blended fuel is going to cause some major issues in our older engines performance. The ethanol blend is going to scour your fuel tank so an additional filter is really necessary between the tank and the pump. All rubber fuel lines are going to need to be upgraded to the blue-lined fuel injection grade hose. All replacemnt parts (fuel pumps)after 1985 are SUPPOSED to be E10 tolerant but I added a supplimentary electric pump at the tank to deal with "phase separation" which is a condition similar to old school vapor lock. If your engine was tuned for straight gas, you're going to have to richen your idle mixture significantly to eliminate a lean-burn condition and the tell-tale popping noted in the exhaust system during deceleration. The only up-side of this mandated blend is that it will help those folks out who can't get their carbs leaned down, and the internals of the engine will have less carbon buildup.
As for the validity of the numbers....if you back them up with actual time on the road and some documented observations you'll be far ahead of the EPA fuel ratings that we've been fed for years. I've taken the time to do just that and am more than willing to share my observations.
Dave
