I am not familiar with the kit that you built that gun from, but I would bet a six pack that there is a lot more you can do to make it look right than just square the bottom and trim the fore-end.
If you could post more pictures of the rifle, we might be able to suggest more fixes while you are at it.
I would bet that the lock panel surround (lock mortise) is way too big. Should be around 1/8th inch or so for that period.
If you only sanded it, the wrist is probably too big, etc etc.
More pictures would help.
That cheek rest looks like it needs some work as well. a side picture would help greatly, but it looks quite long, and the transition at the rear and front of the cheek rest is not quite right to me.
As for squaring up the bottom of the butt stock, looks to me that you would only have to remove 1/4 inch or maybe 3/8ths. You could probably do that without ruining the lines of the rifle. RCA 119 might have a rounded bottom, but that's probably only one gun out of the entire two volume series. In other words, its likely the less than 1% of the rifles built at the time.
You would probably have to bend the trigger guard a bit to make it fit the new profile, which isn't a big deal. Bend it cold, not hot.
Hard to tell from the photos, but I suspect that butt plate is not really wide enough to fit the general category of Pre-Rev war. They were for the most part quite wide, near to 2 inches.
Regardless of whether you want to make it look like a specific period or just make it look more like a genuine Longrifle from late 18th, early 19th century, I'd be willing to bet that there are a lot of things that can be corrected to make it look better.
Cheers,
chow