Engine running away after carb rebuild....

Here's the deal;
Air entering the intake from under the throttle blades will not cause a runaway............ wait for it............................... because there is no fuel in that air! Too much air entering under the plates, and if the throttle is set right in the first place, will just cause a stall.

Only too much air on top of the plate can cause a runaway, cuz it will pull fuel from the transfer port. And that can only happen if the throttle plate is not closing properly, or if it is opening by itself!
But in neutral, A whack of timing will also cause a high rpm.

So check your Vcan plumbing and your fast-idle cam, make sure the throttle return spring is properly installed, that the KD return spring is properly anchored, and make sure there are no big holes drilled thru your throttle plates. And yes if she's fat in the first place then a leak into the PCV system can also cause an elevated idle, but not a runaway!
Good luck