Bud White is correct--the 2-pin models should not be converted to .357. Only those .40 Glocks with 3 pins can take the pounding. That is a fact. All .40 Glocks produced for the last few years (and all Glocks in general, actually) are now 3 pin. There will be a Trigger Pin and Locking Block Pin in the middle of the frame above the trigger, and a smaller Trigger Housing Pin in the upper portion of the backstrap. You'll know when you look at it.
Yes, any .40 Glock (3-pin of course) can be converted to .357 SIG by getting a replacement barrel in that caliber. The barrel needs to be for the specific model because of differing locking block dimensions from compact and fullsize (i.e. a .357 barrel for a G-22 should NOT go into a G-23, etc.).
Stock Glock barrels and almost all after-market barrels require NO fitting--totally drop-in. ALL stock Glock parts are that way, one of the nice things about the Glock system.
Slghtly different to make a .40 Glock 9mm, this has to be specific CONVERSION barrel.
And BTW, most of the time Glock .40 mags will feed .357 reliably. If not, just get some .357 mags--Glock mags are cheap.