You LN3 guys know that there'* a restriction of sorts in the rear of the LN3 exhaust manifold/header, past the oxygen sensor. Its something like a 20% reduction in the size of the exhaust. I have a couple degrees of knock retard...would removing this restriction with a die grinder or hole saw help at all?

Moved out of 87-91 because you don't have anything that will even remotely compare to an LN3 in the Mechanical sections, and your KR is a direct result of the top swap.

followed your monster creation a while back great work on that. I also have a LN3, there is deffinate restrictions in the manifolds(I had seen a set laying around at a friends one time), and with your setup you could deffinately use that extra flow. I will be taking mine off, and porting them. Also I am going to get them ceramic coated so they no longer look like garbage and produce less heat. I don't know if it will fix your problem but with what your running it would deffinately help. just my 2 cents anyways good luck!

My bad Will, I swore I posted it in Forced Induction, but P&B is good

Opening it up sure isn't going to hurt anything as long as it'* structurally ok.
Since no one else here has a ser1 L67 top swap on an LN3, no one can really say for sure if it will get rid of your KR.
It will probably help but who knows?

I know I can be difficult, having the only known sueprcharged LN3, but typically, do better flowing exhaust manifolds reduce KR?

That depends on how much KR you have, how bad the restriction is, and what the root cause of the KR actually is.