The OCC wire is nice wire, but it doesn't like to be twisted very tightly. 21 GA should be fine for the build, but I would still use our heavy duty green wire where it is placed in the kit. I can't comment about the Symmetry Cu cable, you'll have to buy it and give it a shot.
The binding post holes are 1/2" each and 3/4" apart. I believe the RCA jack holes are 10mm.
I personally wouldn't bother with the Mills resistors in place of the stock ones, as a little inductance in a capacitor bypassed cathode resistor is not something I would consider a problem to be fixed.