IMO, it's the barrels....coupled with the desired balance....that determine what the final weight will be. If you're not going to alter the original barrel weight, there's only so much you do elswhere. You can start with a 4 lb barreled action and make a stock out of styrofoam if you want. You can make the weight anything you want, but that doesn't mean it's going to handle well.
Also, IMO, a smallbore Fox that balances on the hinge pin is going to be pretty muzzle light.
What do your barrels weigh? I have a normally configured Philly 16ga with unmarked 28" barrels that weigh 46 ounces. The complete gun weighs 6 lbs 2 ounces with a 1" rubber recoil pad and it still balances 1/4" ahead of the pin. I've no doubt it left the factory at 6 lbs even & fairly muzzle heavy.
Based on that, I'd say if your barrels weigh 47 ounces, you could build it to 6 lbs effortlessly. For each ounce of barrel weight deviation, add/subtract another ounce in the butt.
So, very roughly,
45 oz barrels ~ 5# 14oz
48 oz barrels ~ 6# 2oz
49 oz barrels ~ 6# 4oz
