Help! I need a good mechanic!

If you never upgraded the fuel hoses, most likely the two on the suction side of the fuel pump are collapsing and choking off the fuel. I had that on a road trip in 1991 when they started putting ethanol in regular gas. The engine would start stumbling above 55 mph or uphill. Ran fine otherwise. That was a 1969 Dart w/ 225 and Holley 1 bbl. If you have the later, they give problems even when "rebuilt", but usually at idle from clogged passages in the idle metering block that isn't rebuilt. If the earlier Carter BBS, no such issues. Only use newer "fuel injection" rated hose.

A vacuum gage on the intake manifold will quickly tell if an exhaust plugging issue. If the vacuum slowly decreases (i.e. pressure goes up) as you suddenly increase rpm that indicates back-pressure is building up in the exhaust. If your neighbors don't mind, you could unbolt the exhaust and run wide open and see if it goes above 30 mph better.

Finally, verify your coil isn't over-heating (i.e. paint melting, too hot to touch). If the ballast resistor is bypassed, it will and will start mis-firing at higher throttle.