1 1/8oz proof will equate to like, 2 1/2 in shells.

1 1/4oz proof will equate to like, 2 3/4 in shells.

Boxlock, sidelock, shouldn't matter, but I would go boxlock
as it is not a Holland and you aren't trying to build one.

Damascus, I'd try for steel but also look at how the barrels were made more than anything - chopper lump etc etc