I would call ADCO or check their websitehttp://www.adcofirearms.com/shopservices/
to see how much they would charge to cut and rethread the barrel then pin the kx3 on. They will also charge an assembly fee since they will need to build it before they pin the flash on.
$40 to remove old flash, $65 to cut, rethread and crown, $30 to pin new flash, $50 to take the upper apart and rebuild it. $185 total plus you loose the old flash, must buy the new one and the 12" rail and the receiver. Thats getting close to $500 from my SWAG.
The bad thing about pinning a brake on if you can't do the work yourself everytime you want to change something it will cost you. If you can cut/grind the old flash off you can save a little, if you can drill an 1/8" hole in the new flash and half way into the threads without breaking into the bore then cut an 1/8" pin drop it in the hole and tack a bead on top that will save you some more.
Don't see anything wrong with what you want to do and understand why, just quite a bit of money for what you may get out of it. Sorry for being blunt.