Pintos seem to like alot of ignition advance due to their old 2 valve head design.

The flatness at high rpm may be down to lack of ingnition advance or as chip says it could be your cam choice.

Your det cans will not be much good on a n/a engine as i doubt the ideal ign advance will be det covered.

The only real way to tune ign advance is on a dyno where you can find the lowest advance figure that alows you to achieve the most torque at each point.