First, I'd not assume you have a problem with false knock until you conclusively rule-out detonation.
Second, unless you subject your LT5 to frequent engine starts without warm-up, you're using a plug that's too hot for an LT5 and running too hot a plug at WOT, make the engine more prone to detonation. Unless the engine sees a lot of starts without warm-ups, you need to run either an NGK 7, a Denso 22 or, if you're familiar with the old ACDelco numbering system, an AC 1 spark plug. FYI, the original LT5 plug was the FR41LS, but that was changed just before introduction for the 1990 model year to the FR2LS because of concerns about pre-delivery fouling.
Now, false knock...it is true that the LT5 can have problems with that and such problems can be aggravated with the installation of headers, but again, I'd not assume false knock until you determine the engine is not detonating.
It was suggested earlier that you acquire a scan tester which takes data. That's a wise purchase. If you see 5-deg KR near peak torque, in warm or even 75° weather, your coolant temp is in the 190s, and your running 91-oct gas, it's probably legitimate detonation.
One way to gain info on whether or not the engine is detonating is to increase your octane just a bit by adding a little 100-octane unleaded racing gas to get the octane of the fuel to 95-96. If you up the octane a bit and the KR either is significantly less or goes away, you had actual detonation and not false KR.
Do not run straight 100-octane unleaded as that often prevents the engine controls from passing the knock sensor test and when the engine can't do that, it sets a code.
You can find blending charts for 91, 92 and 93 octane at
http://www.rockettbrand.com/techsupp...taneblend.html
Good luck!