Header recommendation, 1968 Dodge Dart, 340 4speed

Doug's or TTI's. Only one's worthwhile for an A-body. Doug's give you about an extra 3/8" to 1/2" of ground clearance vs the TTI's if it matters. You know, in case you're "ghetto low ridin". Or in case you've actually set your A-body up to go around corners faster than the average dump truck. Because that requires lowering the CG and improving the suspension geometry, which means lowering the ride height. Darned physics.

You'll have to modify the Z-bar for both Doug's and TTI's. It's not hard, but you need to remove the lever arm that goes to the clutch fork adjusting rod, turn it around so the stud for the clutch fork adjusting rod is on the opposite side, and re-weld it to the end of the Z-bar. I did it for the 340/4 speed in my Duster, I run Doug's on mine. Got them from AutoZone with the 20% discount.

Here's a diagram of how to modify the Z-bar.