You should read this: http://www.profblairandassociates.com/pdfs/RET_Bellmouth_Sept.pdf
As for runner tuning you can calculate with some degree of accuracy what rpm you get a tuning effect in your runners using RPM for peak torque =
642 x c x [ SQRT (S/[L x V] ) ] x [ SQRT { (CR-1)/ (CR+1) } ]
= 218,280 x [ SQRT (S/[L x V] ) ] x [ SQRT { (CR-1)/ (CR+1) } ]
where c = speed of sound (remember to account for IAT), S = runner cross-sectional area, L = runner length, V = displacement per cylinder, and CR = comp ratio.
This is a simplified formula, as the plenum also acts as a resonator and has an effect- I'll try to find the formula for that, I have it somewhere. You can get within about 500 rpm with these calculations, at least that's what I've seen comparing it to 1D engine simulation in Ricardo WAVE. The other thing engine simulation tells you about is secondary resonances that change the VE at other rpms and can have a large effect on overall torque curve shape.
I haven't seen the inside of that plenum before, thanks for sharing. It's hard to guess what's going on inside a plenum without 3D CFD- there are vortices that can pop up and ruin air distribution or standing waves at certain rpms, but if you copied the stock shape somewhat that's the least risk, you'll probably be fine. My favorite plenum is the one Audi used on the LMP R8.