225 Dart chugging past 70...

Revving while sitting still it will not use anywhere near the same amount of fuel as running to a given RPM under load, and the wind resistance at 70 mph put a moderately heavy load on the engine and requires a fair amount of fuel. Revving and seeing fuel pressure @ 6 psi is not replicating the situation that you had under load at 70+ mph.

So, if it acts like it is running out of gas, it probably is. Your symptoms point to that. Time to get into the tank, and also check the fuel line all the way. Someone here or on /6.org reported finding a bug stuck up in the fuel line at one time.... Since you changed the carb as well as not reporting any problems under hard acceleration, that tends to rule out the power valve.