Most of the Metric dovetails are 9mm (.354) making them smaller than the standard 3/8 inch (.375) dovetails.
Track of the Wolf has several different rear sights with dovetails slightly larger than the 9mm slots but many of them are rather low.
They also have a lot of rear sights that have dovetails that are considerably longer than 3/8 inch.
All of these sights are castings so the dovetails that are cast into them will require a bit of work with a file to clean them up.
If I were doing this change I would either buy a #RS-PA-15-W Jim Bridger rear sight with a as cast .372 dovetail or a #RS-A-41 "Tall buckhorn rear sight" (.358 dovetail).
Not being fond of the buckhorn sight I usually file off the horns leaving a nice flat top on them.
One of the problems with replacing the big box adjustable sights on factory muzzleloaders is the height of the standard sight blades.
Many of them are very tall so they have an equally tall front sight mounted on the rifle.
If you plan on replacing the rear adjustable box sight you might want to think about buying a new lower front sight to go along with it.
Then again, filing down the height of a tall front sight isn't hard to do.
As to the question, "Should I file the sight or the barrel?" I'll say, always work on the least expensive part. In this case, that would be the sight(s).
Both of the sights I mentioned have a dovetail with the front side exposed. This will allow the use of a fine tooth flat file to shorten the dovetail if it is needed. Just keep the same 30* angle that's on the sight.
That alone will save the cost of buying a triangular file and trying to "kill" one of the flats teeth.