I know I have a lot of questions, but heres another one. On selecting headers, what should I look for? Are short better than long? What size tubing should it be? Its a 340 that I'm gonna stroke to 416. I plan on using it for street/strip. I want the most power I can get, but I also understand that you can have too much, although I don't know why or how much is too much. Thanks.  

1 3/4 are ready available, hedman apprently have 1 7/8 which would give a little more top end charge.  

In the original I meant you can have too much header, like too little back pressure. Not too much power. You can never have too much power.  

Back pressure is what you need when your running under the engine/cams power band.

Wonder why racers don't have tiny pipes/mufflers and instead have giant pipes and open headers?

Cause,They don't intend to race under their power band/torque curve.  

By "under the power band" you mean if your band is 3000 to 5000 and your at 2000(ex.)? What happens if its too little back pressure?