Chris,
I've seen similar questions here a few times, but I haven't had time to reply to them before.
I've seen an article in I believe Rock and Gem magazine about how to go about this. Disclaimer: I have not had time to try it!
The author used a slimline kit. Since drilling holes that long in rock is challenging, he came up with another idea. He cut his rock into thin slabs (with a wet diamond saw) and glued them basically in a square (use the 24 hr. epoxy) around the tubes. Where the edges butted, they were lapped flat, but not polished, so that the epoxy has something to grab. After giving them several days to set up and develop all the strength in the epoxy bond, he found a steel rod that would pass through the brass tube from the kit. Then he used a lapidary grinder (wet) to knock off the corners. Once it was sort of octagonal in cross section, he started to let the blank spin on the steel rod as he pressed it to the grinding wheel. Apparently he was able to get the blank pretty round this way, and taper it to match the fittings. It was sanded and polished in a similar way. I believe I recall him saying that he held the blank at an angle to the wheel, i.e. maybe at a 30 deg. angle to horizontal, to reduce the tendency for the blank to just spin really fast. I think he was also using finger pressure to slow the rotation of the blank.
Note that all the grinding and sawing was wet. Done wet, dust is not a problem. Even after it drys, it turns into cement, so there's still no dust.
By the way, if anyone gets into grinding rock, don't dump the used water down the drain. The rock dust will also settle out in your pipes and solidify there.
Mike