The Keihin CVK34 carburetors used on the 81 KZ750 have more performance potential than do the VM24 carburetors that originally came on your bike. Both will will work fine and the bike will run fine with either. The main problem swapping the larger CV carburetors is that you will have to fit a new throttle cable, intake manifolds and aftermarket pod filters in order to use this assembly. This can be costly. Since you are using pod filters you will have to rejet. Rejetting older Keihin carburetors can be problematic because they use a primary and secondary main jet and the primary main jets are difficult to find in larger sizes. You will also need to use larger pilot jets and this style pilot jet is not to be found. It would be easier to swap out a set of BS32 or BS34 carburetors since the jets are easy to find but you would still need the cable, intakes and pods. The pods and new intakes are needed as the OD of the CV carbs (either type) are far larger than the VM24 carbs and the new cable is needed because the cable end that connects to the carbs is much different. I wrote a "how to" for installing CV carbs and it is in the Filebase under non-model specific tips.
wiredgeorge Motorcycle Carburetors
Mico TX www.wgcarbs.com
Too many bikes to list!