Throw my experience on the pile. I suffered under the same problem with my 68 130. I tried *everything* including new ignition, new fuel line, hard fuel line between the pumps and carbs, different manifolds, different fuel pump, different fuel filters, new gas tank, and multiple carb rebuilds including new needles, new throttle shafts, new floats, new float gaskets, and different butterflies.

Based on the advice of a certain guy who will remains nameless (my unwanted "nemesis") I even pulled the head and tried two different cylinder heads looking for burned valves. That was a waste of $50.00 worth of head gaskets.

The problem was in fact a two-fold issue.

Firstly, the carbs were way over-rich. I tried adjusting and balancing them dozens of times before finally pulling the vacuum hose off the balance pipe while the engine was going. The extra air should have leaned the mixture to death and stalled the engine. Instead the engine raced and stayed high. There was way too much fuel in the mix and a massive air leak would compensate for it. Dialing the mixture down got me up to about 16 mpgs.

Finally, when I went to check the rear, I pulled the drums and found dust and almost no brakes left on the shoes. Seems the self-adjusting mechanism adjusted the brakes to death and cause just enough drag to wreck my mileage. Weirdly, I never felt the drag while driving or rolling the car...

Getting new rear brakes (and later a whole new rear end) got me up to just over 20 MPGs. Still not wonderful, but I can live with it for my city/highway commute.