Well, if you're getting a good deal on the RK kit then thats cool. I have just seen quite a few problems with them and most people who have seen them or run them aren't particularly happy--or at least I should say the people that have had them and have wheeled them hard are not happy. Their were two guys last year that swore up and down about how great RK was, then after about 6 months of that, they started breaking EVERYTHING on the kit--joints, arms, brackets etc. Neither of them stll run the RK kit (well, they run the coils still I'm sure) anymore because of their experience.
as for tires, you can run 37's at 7" with some good trimming, 35's would fit easily. There is a kid in provo that has 37" MTR's on about 5.5" lift with a full bodied ZJ--but he did a lot of custom work on it and has aftermarket axles which help is cause (he was in petersen's not too long ago with a gray/black flamed ZJ). All that being said, your stock axles will not hold up to either tire size--especially if you start wheelin it and especially especially if you lock the axles.
Cody