Contact John Foster in the UK and go to the best sleevers in the business. "$500 to perfectly regulate..." Giggle, laugh...
Foster has done 5 sets of barrels for me over the years-all superbly. However the decline of the $ and the new regualtions that brevent mailing back the gun have made this opertion much more expense. You' still need to find a Parker top rib to get the right job and they may not have sleeving tubes of sufficient girth for such a large set of barrels . This would likely not be cost effective for a project gun.Unless someone like Merrington can repair the barrels, (waiting and) finding a donor set of Parker barrels and having them fitted here is the way I would go (and have gone).