Carb Problems

I have a 327/400hp it has trick flow heads,HEI dist.,Edelbrock 1406/600cfm carb. I pulled the plugs out too look at them and found two of them to show grey color around the tip area looked like a lean burn. The engine has alway's felt like it is holding back is this a case of a small carb or something more.
I was asked about comp. it is the same 155 psi give or take a pound or two

Generallly, 600 is a bit small for 400hp, and may choke your top end.

if 6 cyls look good, and only 2 look lean i'd do a compression test and leak down to verify it isn't mechanical in nature.

you could rejet your 600, or just get a 750 and work with that.

How do you know you are getting 400hp?