I recently put a set of LS6style heads on my car and I already had LS1 headers on the car. Now I've got a D-port head and an oval port header. I tried using a Fel-pro LS1 header gasket but it leaks.

Has anyone run into this? Whats a fix?

 

Doesn't sound like a gasket problem. Sounds like a warped header flange. The gasket mating surface area of the head should work with the round gasket.