Subsequent research showed that he was right. The barrels would interchange, the magazines had the same dimensions, the ejectors would work and the recoil spring assembly was the same part in both guns. Wow! I used to work with Glocks a lot - selling them, shooting them, competing with them. I went to the Glock armorer's school when we owned a gun store. I had just forgotten how much commonality of parts there was in the Glock system.
I checked out different barrels and searched around for parts. I ended up with a Storm Lake barrel with cut rifling and a fully supported chamber, which is all bright and shiny and fits into the cut in the slide very nicely.
After replacing the slide stop lever that I broke while trying to bend it to where I wanted it (it is not for nothing that I was called "Captain Overtorque") I took the whole shebang to the range for a super quick range session. Taking the stock gun I loaded it with Blazer .45 230gr ball and fired eight rounds through it. I then swapped out the barrel, loaded it with 10 rounds of Blazer 200gr 10mm ball and without any other changes to the gun, let fly.
End result - 18 rounds through the gun, two different calibers, perfect functioning!
A couple of days later I got to shoot it a good bit more and found that the .45 ball shot about 3-4 inches high at 25 yards with the stock original sights, while the 10mm ball shot directly to point of aim with the stock sights.
Amazing. Now I can load it up with whichever caliber seems appropriate to the situation, and carry on. Of course, now I also have to get a holster for it, maybe something from Rocketman kydex hoslters. With the .45 load it'll be fine for personal defense, and with the 10mm I can carry it into the woods with heavy loads for use against beasts with fang and claw, and carry a reserve magazine with defensive loads suited to hominids.
Maybe at some point in time I'll send it out to a friend in Kali for a grip frame reduction. I've always been lucky and have had no problems with the way that the Glocks point for me, but since this is already a project gun I might try it anyway.
Whichever, I'm happy. Now I've got that 10mm I always wanted, not just a hot .40 load, but a real zinger!