Actually, the glock .357 sig mags and .40 cal mags are the same exact mag but with different stampings, all you need is the .357 sig barrel and either mag and you're fine; as most know, .357 sig is a .40 cal necked down to a 9mm, and size of the main body of both casings are identical, hence both mags working in either one's place. But with glocks, you have to swap the extractor if you go from 9mm to .40 or vice versa, as the extractors are different between the pistols. A friend of mine did this with his glock 23 and a conversion barrel from Lone Wolf and it worked fine. There are several companies, such as Lone Wolf, who make conversion barrels for a decent price.
