Keeping water out at wheel wells?

I did not have to build wheel wells for say but where my floor and wall meet is right above the center line of the wheel. So that only gives me the half inch of plywood floor to glue and screw to. (Not enough clearance for any thing like a fender.) Should I add a 2x2 support on the inside 3 inches past each fender well ? and seal it with? I was thinking of glassing in the the in side seam. I cant do anything to the out side seam because it already has tar on it
Give me some ideas !!!!! the walls are going up today!
Thanks!
Jason

Thanks!
Jason