If you are not using the hex shaft, then you could use some all thread or a long bolt with the spacer tube to separate the plates. Center drill both ends and turn the cluster on centers or chuck the front plate in the lathe and center the rear end with the tale stock.
The shaft won\'t slip if you snug up the nut good.
The round shaft with a cross hole drilled in it for a dowel pin was what we used for the .357 shafts in that build. I didn\'t realize you were using that type of shaft on your RG-G\'s, it should work well for it with about a 3/32 diameter dowel pin and still have enough shaft material to support the cluster. On the .357\'s we have a 1/2 inch or 5/8 diameter shaft with either a 1/8 or a 3/16 diameter dowel pin on mine, I forget now but you get the picture.
bruski