Re: New neck for older Gibson SG: who makes replacement necks?
The biggest issue with major modifications like replacing a neck is this -
Replacing a neck can be a bit of a gamble, as no matter how well you can define or reference your goals in shape and feel, you can't ever be 100% sure of how it will actually feel and sound until the work is already done and the money is already spent. Changing something so large as the entire neck is replacing half of the guitar - it can have significant effect on the tone, and you're really only bring half the "mojo" of an old body to what is essentially a new custom parts build. It's a fairly large investment, and even if you have a very good idea what you're going for, you're not going to be guaranteed 100% satisfaction on its return.
I generally recommend putting a good effort in to looking around at existing instruments that you can play, feel, and listen to as they are. If you find one that really works in it's current state, you know exactly what you are getting before making the investment, and after sale or trade of your existing guitar you will often come out with less cost in the end. If you search and search, and just can't find an existing instrument to meet your needs then customization can be a reasonable next step. I just think it's generally wise to leave that option as Plan B.