Does any know if a Zr750 f/wheel will go into a set of zxr 400L forks?if so,will it need modding etc...thanks guys

You can usually make any wheel fit as long as there's room for it - front wheels are easy on this score as the zxr runs a 120 front tyre on a 3.5" rim so any wheel will have an identical dimension at it's extremity.

Basically the three dimensions you need to concern yourself with are: disc diameter (if larger than the 400 discs you'll need caliper adapter brackets made up) disc track - ie distance between discs (if narrower discs need to be spaced, if wider hub may need machining down or caliper brackets for previous step can take this into account) and finally the spindle diameter (bearings may need to be swapped for smaller internal diameter - I doubt the 400 spindle is bigger than the 750). Whatever happens you will be needing spacers for the new wheel, but that is something you sort out once the other three dimensions are resolved.

Fortunately kawasaki tend to stick to fairly set formulas and you will likely find the disc track is the same as the majority of 3.5" fronts use an identical track. The disc diameter may be more for the 750 though.