Don't fall into the trap of thinking it will make just as much power without the headers....... It won't.
This^^^

Better heads: definitely, more flow without extending cam duration.

What will help, but it's a PITA (if you can fit them at all): 1" thick steel exhaust flange as a spacer between the manifolds and the head (with longer studs). If you can't water-jet etc. you could use 3 X 3/8" common flanges each side.
1. straighter path for gas leaving the port before the turn
2. more volume inside the casting
3. keeps the pulses separated longer before merge
Yes, 6" thick would be better, but...