Well all you really need in the kit is a new barrel, appropriate length mag tub and a shorter spring. Nothing you cant really source from a Mossberg dealer...
I see the kit is $150 on brownells. For $320 or so you can get your self a new complete Mossberg 500 with the length of barrel that you want.
Might be worth the extra $150, that way you won't have to switch and you can never have enough shotguns.