I don't know if this helps or not. But my Zabala action that I am building the 450#2 on was origionally proofed at 1370 bar. It has just passed reproof at 3500 bar. I think the Rossi or CZ should be fine, however I would stear well clear of any Chineese copy of anything if their DVD's and handtools are anything to go by!!!

As to the sleeving that is exactly how mine is done.

I would have a concern about how the firing rins are bushed and vented on a hammer gun as a reptured primer will have a lot more high pressure gas venting through here.
For information. The newest batch of Bikial hammer guns (here anyway) the hammers are fakes. In that they cock the internal firing pins and fly forward as the trigger is pulled, but they do not have external strikers.

At the end of the day on relativly low pressure cartridges it will be the breach thrust that breaks the action not the hoop stress in a modern steel barrel inside what is left of a modern shotgun breach.

If the action of the Rossi is marked with two five pointed stars within cogs (reenforced proof) and the letters PSF
it is proofed to 17,637 psi or otherwise 14,223 psi.
If it is marked PN it is for black powder only.

For the CZ it is 14,223 but before the barrels are struck up it must pass 15,645. In fact Chech guns must pass proof 4 times.

So punch in these figures, work out your base diamiters to give breach thrust and once it works on paper get out the hacksaw :-)

As I read it the breach thrust of a 303 Briish would work out about the same as 30-40 Krag because the base is smaller at about the same pressure and is ballistically superior. You could get the reamer maker to make the reamer with a neck to suit 308 heads and the reloading die manufacturer to make a tighter neck on the resizing die accordingly. Better cartridge IMHO and better suited for a double gun.

Regards