I ran two Mono-jets on a 250, everything was set right on the money, even the linkage was set with indicators so I knew they were even. I got about 5 miles per gallon with that set-up. It was so bad I could almost see the fuel gauge moving as I drove. The problem I had and what you have, is the air velocity through the carburetors. The carburetor bores are to big for the amount of air required to run the engine. You need two carburetors off a 216, both identically set and metered. The bores are smaller so the air speed is quicker which makes the signals closer to what the carburetor is calibrated for.
A lot of guys are running twin Pinto carburetor, a progressive two barrel with each half having very small bores. I will bet yours runs pretty good at full throttle, peak rpm, and even half decent just driving, both due to air velocity being higher.
So, to cure your problem, you need smaller carburetors, nothing else you can do. If you block one, you will lean out the far cylinders. If you run progressive linkage, same thing, the farthest away cylinders will run lean till the second carburetor is opened. No amount of jetting will lean it out.
I run dual Carter W-1's from a 216 on my 250, It took quite a bit of tuning to get it right, but it was worth the trouble to me. It runs much better then even the stock set-up the engine came with.