If you use a u.s.d ZXR front end try if possible to go for the 'R' model for the 750 or 'SP' for the 400. As they have an additional adjustment in the form of a dial located on the same casting the axle goes through and face backwards. On the standard fork there is just an unmachined plug in the casting.

On older ZXR750 front ends i've seen, sometimes the chrome worn through, so i'd check that before buying it as some of these front ends are over 20 years old. Not sure why the chrome fails, it may just be natural wear.

The spring rates will be completely wrong, regardless of playing with the dampening.

I'd be interested in the weight difference between ZXR and NSR forks, both in terms of sprung and unsprung mass.

I think at the time (early 90's, or 89 onwards for the 400) these fork represented a major leap forward, and this contributed to ZXR's reputation of being planted at the front.

Still for the amount of time it takes to change the front end over i'd spend the extra money and get later model front end with radial brake calipers even if it is mainly for aesthetics reasons and also i suspect the later the model year the easier you may be able to find something off the shelf to resolve the spring issues.