There's a lot more to it than just making the hooks fit the tang. It is nigh impossible to fit an original tang to a gun it wasn't made for. You will have to have the whole setup custom made to match the barrels you are using.
I completely disagree .
Like anything else , the part must be fit to the original . Its not hard to do , just time consuming
I have made many a SXS through the years and I don’t recall every having a set of plugs that matched a Standing breech .
Now I have made plugs to match original standing breeches . I have even made standing breech’s to match original plugs . I also have re fit originals to fit different barrel sets . To include modifying percussion SB to fit on flintlocks .
mating an original standing breech to a breeched set of cartridge barrels
here is another were i used a long tanged SB to fit to an original set of barrels thats matching SB had been broken off
I will also agree that all lock ups are not the same . The reason they aren’t is that for the most part they are mad to fit the barrel set . Not the breech plugs made to fit the SB .
Take you time . Inletting black is your friend . When properly made the barrels will fit and lock up properly .
Also not that not all designs are hooked . Some are post with locking bolts. Here is a early French with such a standing Breech design .
Still others only have one hook . In the case of this stepped breech SXS the hook is center . However I have seen original applications where this single hook was as wide as a double hooked breech .
standard double hook. this is from an original Parker