I disagree- the .40 would be the best barrel (blank) to start with. That way all the external dimensions match the .40 frame. The assumption on my part has always been that the barrel blank would be drilled, chambered and rifled for the .357.
OK, so what you're saying; you are going to buy a Barrel Blank for a .40cal CZ75B, that hasn't ever been bored or rifled in any caliber (maybe (just maybe) CZCustom can supply that to you)
Then find a Pistolsmith that with bore and rifle it to 0.355, and chamber it in .357 SIG (maybe CZCustom will even do this for you)
I'm saying this with all the best intentions.....GOOD LUCK with that !
I think you'd find that the standard (NOT an SP-01) CZ75B 9mm Barrel and the standard CZ75B .40cal Barrel both have the same physical outer dimensions
But I could be wrong
If you call CZCustom or PM Stuart here, he should be able to clarify it all, and give you same solid advice on your best path to take
Again, good luck in your quest