Hi,
Yes, it has to be drilled at an angle to reach the center barrel powder chamber. I assume you have all the holes drilled from the central powder chamber to each outer barrel? On the originals, those were drilled from the outside of each barrel all the way through each barrel and into the center powder chamber. Then the outside holes were threaded and plugged. Based on my very old notes from Kit Ravenshear, the vent hole should be at least 1/16" but 3/32" would not be too large and perhaps more reliable because the length of the hole.
dave