The chain is 1/4" pitch standard roller chain. The drive sprocket was 9 tooth, but don't buy them from buildyourcnc.com because both the sprocket and idlers they sell are total crap quality and will cause problems. I forget where I finally wound up buying good ones though, I'd have to look it up. Motors were NEMA 23, except the motor that spins the tool head which was NEMA 17. I should have just put a NEMA 23 on that one too because when the cutter wheel digs into the table surface the motor can't turn it sometimes and it loses steps.
The rails are not mounted against the edge of the table, so they are not effected by inaccuracies in the table edge. They sit out away from it by maybe 1/4". I used a metal spacer bar to get the rails parallel. I believe I installed one side first and then used the spacer bar to align the other side before screwing them down. I also ran the gantry back and forth to make sure there was no resistance or binding as a double check for the parallelism before finally screwing down the other side. It is one continuous rail on each side, so I could clamp the ends to keep it from flipping over while aligning it. The mounting holes are not really oversized, so there is not much play.
Sorry for the slow response, I didn't have this thread setup to notify me when people post here.