Who else thinks there is not nearly enough steel mass at the intersection of the breech face and water table for full power black powder loads on this sidelock action? Should the builder, if they go ahead with it, redesign the reciever to include a radius/fillet at the intersection? If they did, the rebuild gets more complicated. Sharp internal corners are stress risers. This would make fitting the barrels more complicated though. If the new receiver is changed to include a thicker breech face, the stock and lock fitment will change a lot of things. I see how precision lock and reciever fit and bedding, fat boy stock wood, and choice of quality lumber becomes vital with the big bores. The project is a worthy opponent. Respect!
PS the antique regs say nothing about teatfire either.