HI.
I'll try and help you with both of your questions. I have the Gibraltar Bass mount. The mounting holes are at 4.5" on centers. After checking my R.B. Gretsch rail mount (same as a Slingerland), the mounting holes are not the same ( R.B. about 3.75"). Steve Maxwell Drums is selling a new Rail Mount Assy.....supposed to be the same as a Sling./R.B.
There are a couple of sources for the cymbal arm. One is American Drum Parts(http://www.amdrumparts.com/misc.htm). The other, Drummaker .com(www.drummaker.com). Both sources are selling the same item.
I'll try to post a pic. of the Cym. arm. Hope this helps you out. I'm new here (observing for quite awhile). Still enjoying restorations after 15 years.
DAVE.