You can find 6" barrels all over the place, they too are not cheap. The cost is not in the threading or the cap, though plenty have blown it right there.
Then there is the very real issue of link pin fit, bushing tolerances and the type of shoulder barrel facing your .45 suppressor will run best with. Some shoulder up past the threads, some use the front of the barrel, some both.
Can you save $50 and make it all work, absolutely, is it worth it? Harder to know.
The 1911 is more forgiving in this sense than say a Glock or S&W, H&K, Beretta, Walther, etc., etc. The key word there is "more" most of the times you end up beating it out of something.
Welding on an extension is for those barrels currently not available properly configured. Freeboring right there, prior to the Neilson has consequences, prior to the diverter has consequences. Turning the barrel down, threading both the barrel and the extension, maybe.....
http://lmgtfy.com/?q=.45+acp+barrel+6"