My memory is quite vague on the price of petrol. This site says the price in the States was about $1.10/gallon, and I remember thinking I was paying the same for a liter in the UK as folks were paying for a gallon in the US. If that's true I was paying about $4.25/US gallon in 87/88.
Tax on fuel accounts for most of the disparity. Hopefully nobody is suggesting that confiscatory gas tax rates are good for anybody. Although, it costs a bunch for a carrier group so it's gonna show up somewhere. Probably inflation and higher ... taxes. Hm.
Anyway, one thing to look at with the big V8s is engine tuning - timing and advance, weights and springs, and vacuum advance. This will add tremendous percentage gains at least, compared to a poorly tuned motor.
Racing motors are setup and optimized for wide open throttle, not for economy. Careful tuning will at least get the most of what is technically feasible.
You didn't mention whether you had auto or stick. Converting to OD with an auto will get involved and spendy. Converting a stick will cost you parts and a couple hours of your time. City MPG will be crappy regardless, hwy MPG will improve a bit, but don't expect to see the high side of 12MPG no matter what you do. In fact 12 would be optimistic on a 4x4...
A pre-smog timing set and an advance kit for the dizzy, along with setting the base timing to 12 degrees BTDC will help with performance, but take it easy on the gas and it can help MPG a little... probably not enough to be much help, but a little. You would have to do other things to see any significant improvement, but you'll never see a return on your investment.
Or basically what everyone else has said! (great minds think alike...)
They use a timing chain and sprockets from a old 429 I believe. This changes the valve timing to a more favorable setting.
There can be gains from playing with the distributor, but a Ford dist is a real pain to play with. If you were serious about doing this, I think I would put a MSD setup, dist and all, where you can more easily mess with the springs and weights. It's not cheap though.
Not to hijack the thread, but I'm putting the EDIS ignition and MegaJolt on my engine. Lock down the distributor, and then it's time for some fun! I'm at about 9000' in elevation and the MegaJolt will make it much easier to find the right timing.