Mac:
Pretty sure the one I have seen was built by off-setting the .22 barrels inwards, not top-dead-centre. It didn't look strange at all, since the 'inside' taper is covered by the rib, and not obvious. The outside barrel-taper is slightly steeper, like most SxS shotguns. Also, no need to thread with a .22, just tin them in. Keep it simple...

Cheers.