I had a 68 Chevy do the same thing, didn't take long to figure out there was a lot of trash in the fuel tank, and would stop up the fuel filter, could go 60 easily for a while then as the filter stopped up, top speed would fall off.
Could be the pump just not keeping up with the demand at 50 - 60, but just enough to allow a slower speed.
Check the sock in the tank too.
You are probably right, sounds like it is starving for fuel.
