Ben,
I'm afraid that you would never be able to get 8 gauge rifled tubes, get them installed, and have the gun shoot anywhere close to the point of impact that it was originally regulated for.
That would mean that it would have to be re-regulated ,anyway. A better plan would be to use solder-in, rifled full length tubes and just figure on re-regulating it. If that is done though, it will make the gun lots heavier, depending on the new caliber or gauge of the rifled tubes you choose. The last choice is to cut off the old barrels, make a monoblock out of them, and put in something like 10 gauge rifled sleever barrels with stubs turned down to fit the new monoblock. If you do that, you have lost all the original collector value, but in the condition it is in now, there isn't any real collector value left anyway. By going that way, you could lighten up the finished product and use shorter tubes which would make the whole thing handle much better.