I'm getting ready to build a set of headers for my big block 66 Dart. Motor will end up at either 470, or 512ci (shooting for ~600hp on pump gas, but haven't decided on stroke yet). This will be the first set of headers I've ever built. Is there a rule of thumb for optimal primary length and tube size?

Also, where do you guys buy tubing, flanges, and other header parts? Or are there kits you recommend? I'd rather stick to mild steel.