Tenn,
It looks like you have a plan, and a good start.
I'd think that 13/16 barrels would be about right to keep the weight from becoming excessive on a swivel, espicially if you taper them. I have a late percussion swivel with straight 1 1/8 barrels and it weighs a ton! On the other hand, I looked at a Adam Angstatt with slender tapered barrels, and it was as light as a feather.
Are you going to use a modern swivel/lockup arrangement and barrel attachment like David Price uses, or a more traditional method? I can see where both styles have their merits.
Early wood forearm, or later brass one?
The only advice I can give is to think Slender (top to bottom) regarding the lock, particularly the forward part with the pan and frizzen.
If I was ever going to build a gun, it'd probably be a swivel breech too.
John