What plugs are you using? What carb and carb jetting? What cam and intake manifold? is the thing starving for fuel at 5300rpm? (lack of fuel flow volume at high rpm)
Did you verify true TDC using a piston stop to ensure you are setting the timing accuratly? Vortec motors often don't need much total timing. Some as low as 28deg.
29 to 34 is typical. The AFR where a motor will tend to knock easiest at WOT is often not very lean 13:1ish. You may have to go further richer (even at the cost of a few ponies). Especially if it is one cylinder of the 8 is knocking. May have to stagger jet. You are on the edge as far as cr is concerned. If you want lots of initial timing for idle quality and low rpm throttle response, (long cam duration) and conservative total timing, you'll need to recurve the distributor to limit the mechanical advance travel stop. Look for a manifold vacuum leak. One leaky intake manifold/ intake port gasket joint will cause a lean(er) running, knock senstive cylinder (especialy if the gasket leak is drawing in engine oil into the intake port) Look for oil in the PCV hose. (bad or no valve cover breather/PCV baffling) Look for trans fluid in the transmission vac modulator line. Vacuum advance amount and rate, will need to be carefully adjusted to avoid throttle roll in knock.
Champion RS9YC are colder than stock. Mixing in 110 octane unleaded race gas is cheaper and more effective than use less lil cans of octane booster.