Rebuilding an Olds 350 boring it 30 over with flat top pistions. Was wondering what duration of a cam shaft would be best to use to keep street legal and add horses

For the best answer to that question, I'd get in touch with a camshaft manufacturer - those guys have cutting edge info on that, but here's a simple guide:

More Lobe Separation will provide a wider powerband, more peak power but with smoother idle

Less Lobe Separation will increase mid-range torque, provide faster acceleration, and a narrower powerband

More Duration will move the powerband higher in the rpm range

Less Duration will provide more low-end torque - important if your focus is the jump off the line.

More Overlap will provide improved signal to carburetor (if you're using a carb), lower fuel efficiency, potential for reversion

Less Overlap will provide improved low-rpm responsiveness, better fuel efficiency, and the engine may run hotter