Is exhaust pipe size ID or OD?

Howdy Folks! It's time to display my ignorance once again.

The exhaust on my 69 Dart 340 is all wonky. Whoever put it together got the angles all wrong, and now the turn downs which are just in front of the axle are pointed right at the wheel/shock/brakes which can't be good for any of them. It also is hanging down quite a bit lower than necessary, reducing ground clearance.

I need to get new header collectors, cut the old pipe off, and re-weld it correctly. When ordering the collector/reducer, is the size of the exhaust pipe determined by the ID or OD?