I'm checking your gearing specs. Looks pretty darn high! V8's with the 0.81:1 SROD four speed or 0.79, 0.68 T5 are very economical. My mate had a Chev 283 2-bbl, 350 Holley, with a Toyota Celica 5-speed with 0.86:1 5th and 2.75:1 9" diff in his 2500 pound Holden. He used to get over 30 imperial mpg at 65 mph! If I've read right, you've got the Mustang T5 with a 0.72:1 top, and that means you are running a near stock Ranchero 3.55:1 diff. A/C, power steering. So its got a few power options, but great gearing, especially on the wide open spaces.
I am totally with David and William on this one. If you summed up economy set-ups, it is to have too much advance and too lean at part throttle, then very moderate timing and very rich fuel agumentation under wide open throttle condions. (agumentation = extra gas when really needed).
1. The power valve needs to be changed to a lower number. The carb should run lean all the time, accept when youi stomp on it. Then, the carb should richen up to the fullest extent of the two power valve channel restrictions. These pore raw gasoline into the intake under low vaccum. If you run the stock 10.5 or 8.5 power valve, fuel economy will be worse than with the 4.5 power valve. Remember the rhyme. When the vaccum drops, the power valve pops, and all the champange flows ands your car goes. You need to avoid the action of the power valve to as late as possible. Repeat: The car should be running lean until the pedal is stepped on.
2. The regular vaccum checks and seal of the plugs, Holley's famous leak prone four bolt carb base, the two gromets that cover the two idle jets, the condition of the power valve. It is suggested by most V8 Holley guys that Holleys hould run at 5 to 6 psi. People feel that the Holley is set up for this amount, and why should a good old US carb run on 3 psi like the Europen carbs? Answer is that I've found that in NZ bumpy roads, coomon 12% grades, less than 1200 foot radius bends everyhwere and roads with roughness counts of over 2 times then American roads on the international scale, this creates poor gas mileage. Any rocking motion, and the fuel float just overloads the main jets on a constant basis. On our I6's, unlike Slant sixes and the 240/300, the carbs are the wrong way around. Ford would never make a 2-bbl simultanous carb sit where we place it. Our sixes are worse because the carb is mounted the wrong way around by 90 degrees. The main jet ends up getting exposed at times, and some one decides to jet up becasue of it. Fuel gets heated by the tube headers or heat stove under the carb, and this hurts fuel consumption under fully warmed up driving. Static jetting on a dyno may give 61 or 65, but on the road, you can get away with much less.
Where roads or alignments are bumpy, I prefer to restrict line pressure to a maximum of 3.5 psi, and run the Holley or Summit presure reducer very close to the carby, with a short, large diameter line to the carb.
Unlike a got old 302, our carbs are slung on the side, and pivot up and down about the centre of the engine. Holleys hate being bumped around.
3. Six cylinders have a longer flow path to the outer cylinders than V8's. The 350 is set up for V8 style 2-bbl intkaes, some of which are very well designed. On a six, the squirters can be wound up from the stock 28 to 31. This allows more fuel on acceleration, and allows you to jet back to the more ideal 13 to 14.5:1 under wide open throttle. Outer cylinders run lean, inner ones rich.
4. Advance the cam 2 degrees with an offset key way. 155 psi is too low a compression figure.
5. The standard practice here is to run non extended plugs, wide 50 to 60 thou spark gaps, and a large amount of static advance (9 to 12 degrees is what I've seen here on cross flow and log 250's), with very mild 20 to 25 degrees of advance from the mechanical side. MSD, a good coil. The problem with this is that vaccum advance can add a lot of fuel economy. Everyone wants to avoid the 'big bang' on todays unleaded, so most people drop vacum advance. Our Aussie Bosch Durasparks are not the same as yours. I think the best approach is to run the Duraspark with highly restricted mechanical advance 15 to 20, and another 5 degrees of vaccum advance. Instead, the static timing at 1200 without the vaccum advance hooked up should be about 6 to 9 degrees. This allows the car to reach full advance by about 3200 to 3600 rpm. Wind up the static timing until you get the best second gear acceleration from 30 to 60 mph (on a hill is the best test), and then dial back 1 degree. If you hear knock, wind it back 2 degrees. A lot of fuel can be wasted if the vaccum advance is removed.
Mark top dead centre on the crank, and then 6, 8, 10 and 12. If you can get 14 degrees static advance, then great, but your toital advance then needs to be pegged back. Check the 1200 timing without vac advance, and keep winding it up. This way, you won't over advance. You may find standard timing may be a lot more than what you'd expect, and total advance at 3500 may be up to the ideal 36 to 38 degrees for a well tuned modifed engine.
Hope that helps. Most problems with fuel economy are igniton realted, then vacuum, then power valve.
