The main problem with converted rifles or shotguns is the curve of the pistol grip, especially with reference to the curve of the lockplates

I have been offered converted pinfire 12 bores as original "Howdah" pistols here in the UK

As they (real "Howdah") pistols are so rare I think it is quite acceptable to make stuff up (where the law allows)

A very large calibre handgun is always interesting

Good luck whichever way you go

TH44