October Country's rifle is pretty close to the English sporting rifle pattern. From the photograph on their site, I would make the following changes if I were to build an accurate replica:
I would fit a different front sight. The English rifles used a steel front sight and for low light, would use an pop (swing) up ivory bead.
I would lose the recoil pad, and the mercury recoil reducer. Instead, I would pattern the stock after the original dimensions, which is a far more straighter stock profile. Below, are the dimensions taken from an old (1800's) Westley Richards catalog:
(All measurements taken off the sighting line.)
Drop at comb: 1 1/2"
Drop at heel: 2"
14 1/4" from center of trigger to center of butt plate.
14 3/16" from center of trigger to edge of heel.
14 3/4 from center of trigger to edge of toe.
Depth of toe to heel: 5 3/8"
Cast off: 'about' 1/4", depending on client build.
Width of butt plate: 2 1/16".
These dimensions are correct. I have a number of English big bore originals here and the stock dimensions are pretty much the same. Small differences are brought about from 'fitting' these guns to actual clients (of the guns.)
These stock dimensions give a pretty tame recoil on the big bores - some we shoot with up to 250gr FFg and ball. The stock shape as shown by October Country is gonna hurt you with decent charges.
BTW, I build and restore these monsters, although with modern powders, I tend to draw the line at 12 bores. 8 bores are unnecessary powerful. T Rex died a while ago ....