340 Build

You can get the 450ish HP with the stock stroke, but "Streetable" is a loose term. Also, I hope you're only looking for that power at the crank. If you want that much power at the wheels, God speed.

In summation, you're going to need:
-A good carb. I'd say 750cfm minimum, but don't go overkill.
-A fairly agressive intake manifold. Eddy performer RPM air gap or Holley Strip Dominator are the two most aggressive intakes I know of that are deemed "Streetable".
-Porting and Polishing on the heads. If you do not have them already, install the larger 2.02" intake valves and get an experienced shop to port match them to the intake.
-Pistons for your bore (did you bore it 0.030" over, or is it fine as stock?) with compression upped to about 10.0:1. This is the maximum dynamic compression you will want on pump gas.
-I'm no expert on cams, but I would lean toward Comp Cams, Hughes, or Voodoo. Their techs can usually aim you in the right direction for your combo, or there are several members on here that are pretty good at talking cams.
-Headers. I'd go with Dougs or Tti, and don't bother asking which. Totally up to you. Dougs are cheaper, and cermaic coated.
-Good ignition. Mallory and Accel come to mind, but Mopar Performance makes a good system too. Either way, I'd recommend electronic ignition (I'm not sure if 1971s had switched to electronic ignition yet, but I don't think they had).
-Minimum of 2-1/2" dual exhaust, with an x-pipe.

Most of your power will be achieved through the right cam, intake, and cylinder head combo at 10.0:1 compression. Streetability will be tough at that power, but it is achievable.

Post up what you're working with and I know there are enough people here to help you on your way.

Welcome to FABO, and Good luck!
-Mike