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You could always purchase AN bolts from an aircraft supplier like Aircraft Spruce. They come in a wide assortment of sizes usually in 1/16 inch increments in length as well.
As far as the spacer actually increasing throttle response, that is unlikely as you are increasing the throttled volume (minutely) and therefore the distance the pressure wave has to travel from the butterfly to the valve. Any power increases are most likely seen from the stabilizing effect of the "tunnel" created by the spacer. It would let the airflow return to less turbulent state before entering the plenum. This would improve the plenum filling speed and hence it's efficiency.
Not trying to be argumentative, just correct the whole "bigger volume" theory. The extra volume is at best a side effect. You could slightly decrease the main body of the plenum to counter-act the volume increase from the spacer and you would still see a gain.
Intake harmonics is a very tricky and complex subject and its never as simple as just increasing the volume. A lot more interactions going on here. Also, the need for a "high temp gasket" between the throttle body and plenum is crap. The plenum has a much larger surface area and is in contact with something much hotter (the head). If the "high temp" gasket costs more than the regular one, tell them to blow it out their @ss. That is not to say that a high temp gasket between the head and the intake wouldn't be a good idea, just of negligible use between the plenum and throttle body.
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