Sarg,
I talked to my friend about the shoe lump he is making from scratch and there are a couple of the other problems besides the fitting in of the lump to the underside of the barrels. He is doing a 38-55 on a 12 gauge frame, by the way. He first said that you will have trouble maintaining enough material between the barrel section and the extractor hole, above and between the barrels. If you could manage to do it, your firing pin holes will be off center. He had to lower the barrel stubs and move them from side to side in order to get the firing pins centered. He also built a trial shoe lump just to make sure of the barrel alignment before brazing the actual one to the barrels.
The best way to get a .577 build is to do one on a 10 gauge 3 1/2 inch frame. I have done two of them on these frames by using the monoblock method, they don't taper too much for one thing and they give you enough "meat" to get the barrels and the gun balanced properly. A 12 gauge frame is too small for building a full .577 on. If you are interested I can build you one on the monoblock system that will work correctly and be balanced properly but I don't know what would be involved in getting it shipped to you, it might be impossible to do.
On the .577's and .600's, weight is your friend. Tell me more about your barrels, what size are they and have they been chambered yet? The brand really doesn't matter that much but it would be nice to know if they have been contoured and how much they weigh. Bob