Crossover Tube and Valve Spring Pressure Questions

What would make this cam peak at 5600? I am pretty green when It comes to camshaft tuning and specs.
I was going off of the ISKY website when I ordered it because I was under the impression that a longer duration and tighter lobe separation favored higher RPM. Isky says the range for this cam is 2800-7000.

And on the pistons, I was planning on using Mahle powerpack 4.040 bore -5cc domes during a lapse of brain function. (thinking that 3.91 50 over = 4.040)

Which is why I found those other pistons in the original post now.
I am aiming for 10.5:1 and am still looking for a better piston while not breaking my wallet.

(Edit: I have done more research and I have landed on the Icon .60 over -4.90 dome pistons. They should put me at 10.84:1 at zero deck.)
Because that's what his slide rules and calculators say. People routinely spun STOCK 340s to 7K back in the day. Get a clue. He's wrong.