Tips on tuning new street avenger

That surge in power you feel with a vacuum secondary carb is actually a bog that is the result of the secondaries opening to soon/to fast. The way to select the proper spring is to start with a stiff one and keep going lighter until you get that surge then go back one size.

The rpm range in which the engine makes power is a function of the combination of parts you are using. Getting the carb dialed in will improve the power over the entire rpm range but will not appreciably change the shape of the power curve.

FWIW, I have a 360 with a 670 and XE268 cam and mine has an appreciable increase in power past 3500 rpm too. I think you find that is the nature of the combo.