Fox Creek,
I thunk you could get away with welding the slot first....with a cold rod in the bore that fits snug.......and dip the barrel in water quick after the weld then.......have the new dovetail slot cut shallow as is possible not more than 1/3rd the total thickness of the barrel from outside surface to groove not to land......... know what I mean?
I've done it and had it work alright. If the stress relief bulge was in the barrel it was in a few .0001's in. instead of .001's inch.
Something like you are doing where the weld in the slot would be almost completely cut away any relief bulge would be right at the muzzle and not hurt anything like it could if the bulge was further inside. I should have thunked that one up in round one and not here in round two.
Yer a tough gambler of an Hombre so........go fer it and of it messes up........just cut a lil more off so there's no weld and dovetail sorta shallow and live with it. It would be a good bet.