Well, I simply replaced the plastic inner pushrod with a DuBro one-piece metal rod cut to length, a solder clevis on the servo end and a threaded metal clevis at the control surface.

I tried that spacer business on a Super Sportster MK II I built a couple of years ago. The outer tubes were a straight shot, and I still got some binding. Not much, but enough to cause me to trash that pushrod and use just the wire.

You don't need those spacers. The theory is sound, but in practice simply not necessary, IMO. The outer tube serves as a guide only; 2-56 rods are plenty stout enough to handle the loads on a 40-size model, just needing a bit of support in a few places to prevent bending. The outer tube does that.