Nitro,
Much depends on what you intend to do with the rifle and how you want it to feel.
The size of your 12 g frame should influence your decision.
You need to match the bore size to the spacing between both strikers at the action face.
Otherwise you could end up with a double with very poor handling and/or looks.
The spacing between the strikers will directly affect the barrel spacing at the chambers and at the muzzels and also the overall contour and ballance of the barrels as well as the width of the ribs.
I saw an article in Man Magnum on a 450 cal built on a 12 g frame and it looked silly as the barrel spacing was miles apart at the muzzels and the ribs looked way to broad.
577 is a good size for the 12g frame.
I'm a double rifle shooter rather than collector, so I want a gun that has the right feel, fit, and handling quality rather than a gun with a certain makers name or particular caliber.
As I've stated before, it is not hard to build a double on a shotgun frame, but the end result can vary greatly as far as how the gun handles and "feels".

I've handled a few doubles made on shotties and very few actually feel the way they should.
Hasten slowly on this one if you are hoping to end up with a gun that is a joy to handle and shoot.