...the 351, bored .40 over, heads shaved .30, flat top pistons, mild cam, mallory ignition system, carter competition afb, headers, c5 auto trans is getting me between 9-10 miles a gallon.
Hey desi. Unfortunately, you've got a few things against you getting better mileage right now. Possibly anyway, as, after all, "your results may vary" is a common thread in mileage numbers and Broncos.
You have a larger engine than stock now, with plenty of at least semi-performance mods done to it, and you don't really know if they've been optimized for mileage, power, or not at all. Unless the previous owner has detailed exactly what they did that is. And maybe even then, they were leaning more towards the performance end than the efficiency end. Sometimes they go hand in hand, but not always.
I'm assuming that this engine is quite a bit larger than your Mercedes engine is, by quite a bit, and the Bronco has the aerodynamics of a brick, where your E-series is pretty slippery going through the air.
And the EB has the weight of quite a few more of those bricks to haul around as well!
Each of the mods made to the engine might be good for efficiency. But then again, they might not.
Headers are usually good, but not if the exhaust system they're hooked to is either too restricted, or too open. And if the two banks are not interconnected with a cross-over system of some kind, you might be losing some of your street-efficiency to achieve that cool sound. Maybe.
The Carter 4bbl is good too, but not great. Definitely not a mileage carb on a Bronco. But which one is it? 500cfm or 625cfm? Manual or automatic choke? What about jetting? Have the accelerator pump, piston springs, metering rods, and jet sizes all been optimized? If not, you're potentially losing a lot.
The Mallory ignition might be a good thing. But is it vacuum advance or non-vacuum? Potentially HUGE difference in fuel economy there, if you're not using vacuum advance. Points or electronic? Wired properly and timed properly? As mentioned, check the timing and give it as much as it'll take without pinging. What about plug gaps and wires?
What about the heads? High compression or low? Big valves or small? Large runners or small? Old-school combustion chamber design, or new? Aluminum or cast-iron? That .040" overbore has changed the relationships between the combustion chambers to the cylinder wall by at least a small amount. That can do things like unshrouding valves where they want to be shrouded. Lose some of your quench area and all sorts of other things. Some might actually be good things (unshrouding a valve is usually good, but not always), but just as easily might not. I'm reaching pretty far out here, with the overbore, but there are changes in relationships, so there might be dowsides to some of them. At the very least it's now an even larger engine (by a few cubic inches only of course, but it's there), so there's a possible downside to that as well.
Now to the cam. That alone might be why you're not getting very good mileage, or might in fact be why you're getting the "great" numbers you are. I know there are a lot of 351-equipped EB's here that would love to be getting 10. Even on the freeway!!
As mentioned, what about the speedo and (even more importantly) the odometer? Been checked and compensated for? Worth it just for knowing.
Basically, I'm just saying that your rig might be tuned to within a gnat's patootie and getting the best it ever will, or it might need to be fine-tuned to get more. Or the components may just be so far out of the efficiency range for a vehicle like ours that you'll never get more until you change out some of them.
Just depends on what exact combination you're running.
Or not...
Then again, there's driving habits. Take your EB on a few daily-driver routines that you would normally take the Mercedes on, and see what you're getting then.
I'm really just over-thinking it all as usual, but it's fun to talk about the differences and potentials, and it all makes a difference in the results you get.
Good luck. Be interesting if you manage to find a few extra mpg out of your combination.
Paul
I also have an older 2002 Lexus IS300, that is my commuter now, it gets about 24. If I coast the Bronco to work and then call AAA to tow it home, I can get 30 mpg.. which is nice.. 
