Texean:
One thing to keep in mind -- whichever group appeals to you -- is that the level of authenticity in your attire is a decision that you will be making -- as long as it meets the organization's basic requirements.
In SASS for example, pretty much anything "cowboy" goes. You can go to a SASS match and see the full gamut -- from those with Roy Rogers-style, snap-closure Wrangler type shirts and 6-color boots to folks whose attire is meticulous and far exceeds even the standards for NCOWS.
At an NCOWS match, the same is true -- to a degree. Although NCOWS' basic requirements are somewhat more strict and more historically based than SASS, there are still folks who just meet the basics and those who are also meticulous to the nth degree. Most, like SASS, fall somewhere in the middle.
As to shirts, fold-over collars (no button-downs) were indeed common in the era -- primarily as everyday "working" type attire. Banded collar shirts (which were basically designed to wear with detachable collars) were also very common -- with the collars often reserved for more formal or business occasions. One trait that overshadowed most of the others is that shirts were primarily pullover-style with only a 3 to 5 button placket down the font. Full-button shirts were available (especially later in the period) but were not nearly as common.
Pants are another matter. Typical correct traits include: button fly, high waist, no belt loops, buttons or studs for suspenders, rear pocket only on the right. Old photos (except for farmers and common laborers often seen in baggies) tended to show a fairly slim or "stovepipe" leg and certainly with no creases in the pant legs. In actuality, they differed very little from modified Levi 501 style pants with the exception of material (wool and blends were very common) and the somewhat lower waist on the Levi's. If you're planning on modifying 501s I would recommend black as the "blue denim" color was somewhat associated with miners, day laborers and such until after the turn of the 20th century. (Levi Strauss, as I recall, made his first pants from basic tenting canvas which was muslin or duck colored, before switching to the "serge de Nimes" material we now call denim.)
A substantial percentage of men's pants (save orveralls and pants designed for hard usage) started life as part of a suit, which was standard attire for town dwellers, including lawmen and the business class, gamblers, easterners and the like. It is also common to see period photos where the pants don't match the vest and coat. Some of this (based on fashion) was deliberate, but frequently was due to the fact that pants wore out first and needed to be replaced long before the vest/coat.
The bottom line, IMHO, is that you need to decide how detailed and how authentic you want your attire to be and then pursue it. A little reading, research and asking lots of questions will yield a substantial amount of info for you.
Glad to see you joining the Old West crowd.
Welcome.