I've been working almost exclusively in the civilian arena for most of my living history career, generally in the 1855-1875 time frame, and would like to offer the following for you to consider:
Careful study of period photographs will reveal that many of our notions regarding the appearance of men in that era have been heavily tainted by the efforts of the movie industry, so frequent reference to reliably dated photography or non-tabloid artwork will prove helpful.. With that as a starting point, a Civil War era sack suit, or any portion thereof, would work well for your impression as would almost any other item of civilian attire that you can date to the period 1861 to 1865. If your impression is that of a recently de-mobbed soldier you might still have a piece or two of issue clothing but that becomes exponentially less likely the further away you move from April 9th, 1865. You didn't mention which side of the fence your soldier was on but I would suggest you strongly avoid transitioning any Confederate attire to your impression.
Military boots will work just fine but so will brogans or any other sort of period appropriate shoe or boot, and careful study of photography will show that trouser legs are as likely to be worn outside the boot as they are shotgunned inside the boot shaft, even when spurs are worn.
The Remington would be a fine choice for a firearm, but not everyone in the old west went around carrying a gun; that's yet another legacy of the movie industry's impact of our understanding of an historical period. You might be better served with a long arm, or even no weapon at all, until you begin fine tuning your impression.
Allow me to share a couple of images taken of myself portraying exactly the year(s) you've mentioned:
View attachment 588412
This was taken at Fort Lancaster, in West Texas, several months ago. Clearwater hat, Sekela shirt, and SUM civilian vest. The unseen trousers are SUM corduroys sans waistband. No spurs and my pant legs are outside the shafts of my boots. Time period 1865-66.
View attachment 588413
This shot was taken at Fort Concho, also in West Texas, about 6 months ago. A different beat-up old Clearwater hat this time and another Sekela shirt, along with a SUM handkerchief used as a bandana and the same SUM corduroys. Boots are generic Missouri Boot and Shoe worn outside the pant legs in the first photo snd shotgunned inside them in this one. Please note that the spur straps shown here work just fine for the year I was portraying at Concho (1876) but would be inappropriate for your target date. As an aside, I often wear a belt with a harness buckle rather than braces when my attire doesn't include a vest, and note that neither impression includes a revolver or any other firearm. The devil's in the details, and your material culture opportunities become almost endless once you leave military service!
Could you please post a photo or two of your outfit once you get it put together? I'd like very much to see your take on the time period!
Best,,
phil