I haven’t done what you are considering, but... Would the Volunteer, a generic version of the British military match rifle made by several gunmakers in the early 1860s, fitted with an American scope sight be period correct? Such sights were not permitted for competition use in the UK. The few British rifles, notably Whitworth and Kerr, that were imported to the US for sharpshooter use during the Civil War that had scope sights were insofar as I am aware fitted with the side mounted Davidson scope.
Have you fired a Volunteer fitted with a Malcolm scope? I’ve seen pictures from time to time of rifles fitted out as you suggest and always thought the scope seemed a long way above the stock.
David