1964 Dart Owner - In Over My Head

The stock exhaust manifold outlet is 2 1/4" , the head pipe to the muffler is necked down to 1 3/4" , the tail pipe exits the muffler at 1 1/2".
This makes for a lot of back pressure and a lot of torque at low RPM off idle to about 1500 RPM then it strangles the engine at higher RPM. They did this to make the \6 feel snappy on the test drive.
If you up size the exhaust to 2 1/4" from header pipe to tail pipe and with a muffler with some back pressure it will really wake up the performance especially on the highway.

Using an electric cooling fan will also help performance.

Standard gear ratio is most likely 3.23 , with larger diameter tires you should go with 3.55 at least. The stock tires were 6.50 or. 7.00x13" they were about 24" diameter. Your tires look like they are around 26" This is like putting a 2.50 gear ratio and that will kill performance.