The 6X heads will have no performance advantage over the 7K3 heads. To increase performance on the 400, substantially, you'd need more compression and a cam change. If you keep the 7K3 heads, a 262 Voodoo cam, will increase cyl pressure & performance some.

http://www.lunatipower.com/Product.aspx?id=1775

3.23 gears might be good for a 4-speed 400 car.

And, some 72cc or 74cc alum D-port heads would liven things up a bit. But, they're about $2500 or more, nowadays.

http://www.krepower.com/Pontiac%20Aluminum%20D-port%20Cylinder%20Heads.htm

https://www.summitracing.com/parts/...M69viA4I9CGllmuy5a260cWc1IILgxoCwPfw_wcB

With around 10:1 or more CR, a Summit 2802 or Crower 60243 cam should make good power.

A properly built & tuned Q-jet, and a good ignition curve can improve 400 performance.