Rob,
I've always had my tool and die maker friend (I have a 9"x 32" South Bend lathe but no mill) bore out the chambers on his mill. Frankly, when you look at the cost of big reamers and taps, it's actually cheaper to pay a pro to bore the chambers rather than to buy a reamer. I have three taps now, 15/16-32, 13/16-32, and 3/4-32. Each of them, with shipping and taxes, cost me over $100 CDN. This durned hobby is gonna make me poor! Two books that I find invaluable when planning these projects are Machinery's Handbook, for all of the available threads and their dimensions, and The Gun Digest Book Of Shotgun Gunsmithing, by Ralph Walker. The latter has all of the line drawings and dimensions for the chambers and cartridges in all of the gauges from 10 to 410. Suggest that you use Hi-Force 44 solder and its proper flux, from Brownell's. It has a strength of 28,000 psi and is FAIRLY impervious to hot bluing, provided that you don't leave the barrels in the salts bath for an hour!