The 700 and 800 crankcases are the same part number, so it will fit. There will be less cooling on the 800 cylinders, and make sure you go with the updated stainless steel guillotines on the VES valves if you do the swap.
It's pretty spendy to change cylinders/pistons to make a really good 700 engine into a really problematic 800 engine... if you are looking for more power, gut the air box, add a few vents in the intake, and add an SLP single pipe/can. You will have about the same HP in a modded 700 as an 800 without all the issues with crank failure the 800 is known for.
Exactly, the 800 is just a disaster waiting to happen. Also with the airbox still use it just don't throw it out, modify it correctly like Dan is saying. Also if you do anything with the exhaust do a can but that is just me, I am no fan of them because of the problems that they can cause not too mention people who allow access to their property for trails majority of them don't like the loud exhaust. Just a friendly FYI
I would say just leave it alone and do what you can with clutching, the airbox, reeds and be done with it.
The SLP pipe/can isn't any louder than stock. It has a lower "throaty" sound and is not near as obnoxious as some of the others, like the old PSI pipes I had for my 97 700.. those things would make your ears bleed, but OMG they really woke up that engine!!
The combination of both does make a difference. Heard it myself on a sled up here a few years ago. I am just not a fan of them. We lost a lot of trails because of people being stupid in their "loud pipes saves lives garbage".