You can order SpeedPro pistons and rings for pretty cheap (around $100). If your pistons are flat-top, I'd strongly recommend doing it, as the SpeedPro pistons will have notches for valve clearance.
Make sure the bottom end is in good shape. If you're going to run nitrous, I wouldn't keep the stock bottom, but it depends on how big of a shot you plan to run.
Most of your power will come from your heads/intake/carb/cam. More than 350hp, I'd recommend you invest in a main girdle- well worth the investment as insurance. If you're going to run a cam, I'd recommend you go ahead and do that before dropping the motor, since it's a LOT easier while it's out of the car. The rest can come as you get the money.
If you want my opinion, get all Edelbrock parts- heads, intake, cam, lifters, pushrods, and carb. Their Performance RPM Power Package is amazing- look into it at www.edelbrock.com
and decide for yourself.
Any more questions?