According to the book "The Big Guns, Civil War Siege, Seacoast, and Naval Cannon" by Olmstead, Stark, and Tucker the second pattern 8 inch shell chambered shell gun of 63 cwt was described as having a nominal length of 106 inches, weight ratio, 110.3 with round shot, bore length, 12.8 calibers in early tubes cast between 1841-1843.
There were four patterns of this gun. There are no drawings for the first and third patterns. Your drawing is very close to the second pattern. The only noticeable difference is that the muzzle ring on the original is closer to the muzzle.