dpetryk is right. And since you have everything apart for flushing, replace all o-rings with r134 type using Nylog lubricant, replace filter -drier with correct type, replace pressure switches, and H block with r134 types.
I'm not sure which kit you are referring to. There are kits that have adapters, the correct o-rings and oil. That is part of what you need. Other kits from Wal-Mart etc are simply a hose a few cans of R134 and an oil charge of some sort. These kits are called "death kits" by many. They don't kill you, they just kill your A/C system.
I suggest that you stay with R12 or if you insist on converting, do it right or you will be sorry in the long run. The cost of R12 is coming back down making R12 an even better choice.