My main question to you is Why would you want to convert it to shoot 40S&W? After doing our little 9x19mm+P vs 45acp Ballistic Jell test(in the Ammo and Reloading section) I'm totally convinced that ANY major caliber handgun round will be equally as effective if you 1)Use quality HP defensive ammo and 2)Place your shots on target where they'll do the most damage. SO, it seems like instead of a conversion barrel what you really need is some high quality ammo like Gold Dot, XTP or Golden Sabers sitting in your G19c, not a new barrel in 40S&W.
And before someone says it, YES, I do LOVE my G30 in 45acp but I'd still not feel undergunned if I had to have a G19 in 9x19mm as long as I had good quality ammo in it. As always, JMHO, YMMV.