I'm pretty sure the most you'll be able to do is 2mm over bore and use the old '93-'95 cranks which have a bit more stroke and were drop-ins. The most you'll be able to go is up to 850cc I believe. And I think that's with the 2mm overbore and the older crank. Not sure why you think the motor will be taller tough and you'll have to monkey with the motor mounts?
You'd be using the same block just changing out the internals. With the smaller base gasket, the cylinders will sit a small amount shorter but there should be enough freeplay in the upper frame mounts to take that into account.
I'm not sure that this is the proper forum for your question though, as the ZX-7 subforum isn't very active. Have you been on Redmonkey at all? That is the better place for info as there are a lot of Euro guys still riding and modding those bikes. Also, a good place for some additional info is http://www.webzxr.co.uk/
if you haven't seen that site.
I will tell you that I have a '96 ZX-9R motor in my '00 ZX-7R. It was a piece of cake with the BRG adapter kit. If you've already got 2 motors though, that would be easier than trying to source a B-model ZX-9 motor. They are few and far between these days. The problem with the 750 motor is that you can only do so much with it.