What brand of headers should I go with on a 6.0L. I've already got true dual flowmasters already and am planning on putting in new high flow cats. Any advice is welcomed.

 

What vehicle? Truck, car?

 

I'm upgrading my 2003 Chevy Silverado 1/2 ton 4x4

 

The true duals may not gain you as much as you think. This isnt a small block chevy.
I have seen mixed outcomes on them. As far as headers, there are so many it really depends on your budget.
Everyone is going to say pace setter more than likely, and while they are ok, they will give you fits with changing spark plugs and burning wires and are only mild steel with a black paint.
There is also doug thorley headers which are pretty good.
The other option is to go with a high quality stainless header.
We carry
Kooks
ARH
Dynatech (I recommend these for all the trucks)
Stainless works
JBA

those are the only real brands I would recommend.