Bill - the first thing I thought of when I read your post was the NW Trade gun - I've seen some really nice tacked ones over the years.
My thoughts are worthless . . . . but . . . the 45/70 was a step away from being parted out. You've tried to make it into a carbine that is going to look (part wise) as close as you can - you've added a nice aged, worn patina. If you want tacks . . . I'd go for it. I'm thinking brass that are "aged" - i.e. applied to stock, tarnished up well and high points buffed off - allowing for some darkness around the edge and on to the wood to show build up of dirt (for want of a better word) over the years.
In reality, you aren't going to damage the value of the gun any . . . after all, it is a cobbled up carbine made out of a rifle. I had a cut down that I ended up doing pretty much the same thing to . . . without the addition of the tacks. I made the mistake of putting it out on the table at a gun show many years ago and a guy came along who wanted it for the same reason as you made yours. I wished now that I had kept it.
Whatever you do . . . it's your gun to enjoy and if you decide to "tack it" . . . thee are some very good originals out there to get the "period design" inspiration from. I hope you'll post a couple of photos of it - tacked or untacked. I'm sure it looks great! Good luck!
If a pair of '51 Navies were good enough for Billy Hickok, then a single Navy on my right hip is good enough for me . . . besides . . . I'm probably only half as good as he was anyways. Hiram's Rangers Badge #63