Dear Mr. Gunn- It would behoove you to measure the barrel wall thicknesses of your re-chambered gun at the end of the chambers. Merely shooting some normal cartridges in it is not "proofing" as your use of italics shows you know. There is surprisingly little "meat' in many 3 frame 10 bore Parkers (I've owned a lot of them). Funnily enough, in Capt. Askins' Western Cartridge Co. booklet, he specifically noted the Western states that existing 10g guns should not be re-chambered for Super X 3 1/2" shells. If you want to make a Magnum out of a 3 frame have a set of modern steel barrels made for it like some good soul did to this G grade hammer gun I have
(bottom in the photo-actually "proofed" in the Liege proofhouse) Best of luck with your gun and please be careful