about to build a new xj but one or 2 questions

[hucky]

i already have a standard xj600s which is the first bike i have owned. i wanted rebuild it how ever i didnt want to be with out it. Their for i have found someone local to me that brakes bikes and he has 3 frames and a xj400 frame currently sat available.

other than engine and im guessing brakes etc, can anyone tell me the main differences between the 6 and 400 frame?

then the next question, i will be starting this from new so have to find the parts i will be fitting. engine wise, i have seen it has been covered but i still cant find the answers im after. what other engines will fit but with little work.

i.e. has the fzr600 engine got a better power out put than the divvi and would it fit? or is their any more powerful engines that slot in with out alot of work?

i just want to clear this bit up as if i get a more powerful engine i need to make sure my shopping lists have parts that will be good enough. i.e the brakes.

as soon as i collect the frame i will start my project topic