You say, "this will primarily be a scoped rifle". Assuming that open sights are part of the mix, you might want to consider stocking the gun to your 'shotgun' dimensions, but based on the sight line through the open sights as compared to sighting down the rib on a shotgun. You will probably want to leave a little extra wood on the comb, and toe line, in the event you desire or need to adjust from the noted dimensions. You then just live with the lower comb when using the scope sight. This will allow you to use the gun as a stalking type rifle when the open sights are used.

Typically, rifles are stocked shorter than shotguns by approximately 1/2" to 3/4". However, it is a personal thing and I would recommend that you have it stocked long, and then cut it shorter if you find it uncomfortable or unwieldy.

You sure can use your shotgun dimensions, but in a slightly different way...

TRL