the best way to do a front end swap (well at least I reckon it is having done a few) is this:
1. Decide which front end you want to use, and buy it. Complete, wheel, brakes forks and most importantly triple clamps.
2. Either using your OEM triple clamps (or maybe a spare set of clamps if you want to keep the OEM ones just in case) - take the lower triple clamp and have your fitter mate take it into the shop and press out the OEM centre stem.
3. At the same time, take the lower triple clamp from the other front end to the fitter - and do the same thing ie. press out the stock centre stem.
4. Then - either press or weld the original ZR750 centre stem into the lower triple clamp of the new front end. It is really important that at this step - you ensure that the installed height of the OEM centre stem, is *exactly* the same size as it was, when it was installed in the OEM ZR750 lower clamp. It has to be exactly the same installed height - so that the bearings all sit in the same places on the stem, and you can tighten up the locking nuts at the top of the stem (under the steering head) correctly.
At the same time, make sure that the top triple clamp of the new front end will fit over the top of the OEM ZR750 centre stem. The top clamp may need to be bored out a bit, or you may have to machine a bush so it fits the new top clamp nice & snug. This is really important. It has to be a good fit in the top clamp...
5. Fit new ZR750 steering head bearings to the new triple clamp. The bearings will be OEM ZR750 bearings, because you haven't changed either the frame bearing size, or the stem bearing size. All you have changed is the other bits - the important bits that the forks will now bolt *straight* into.
Thats basically it. You can now bolt the forks in - then the wheel and then bolt the brakes up. All these things will bolt straight up into their stock positions - because you are using the stock triple clamp *position* for the forks. The only thing changed is use the original centre stem which obviously fits your stock frame - as long as the installed height is 100% correct. Minor issue is the steering stops. You may have to modify the stock stops on the ZR750 frame, or the stops on the lower triple clamp (depending on what front end you ended up using).
The only other thing to keep in mind is the handlebars. Stock ZX6R top clamps will not have the bar mounts like the ZR750 top clamp does. This means using either raised clip-ons (a good cheap option) or using a modified streetfighter top clamp from somewhere like ASF - these guys make top clamps with bar-mounts for heaps of sportsbikes. Using one of these will let you bolt some stock tube bars on the top clamp, but you will still have to make sure that the centre hole is the right size for the ZR750 centre stem.
Swapping front-ends from one model to another is heaps easy as long as you think it through. It may well be that a stock early ZX6R set of triple clamps may in fact not be that much different to your ZR750 clamps. What you should research in the first instance, is things like steering-head bearing specs. If the bearing specs are the same - that is half the battle. The only other real issue is the length of the centre stem - the only way you can find this out is measure your stock one and see if you can find a donor set of clamps in the local wreckers and measure them up.
Robracer did a front-end swap on his Z750 and used a Gixxer 1000? front end. There is a good thread about this, it was a year or 2 ago, worth having a read of this thread. Good luck and keep the updates coming