I run a 600 on my 340 also. If you're not interested in strip performance or concerned about starving it at the top end, those simple online CFM calculators would probably put you at 650 or less.
It does go at the entrance to the tank. Once the gasket was in place, I was able to get my filler tube in by twisting it back and forth and turning it while exerting downward pressure. It took about 5 minutes to get it in all the way.