The 810cc head gasket will probably work. I previously posted that you'd need a custom head gasket, but now I'm pretty sure I'm wrong.
I'd run stock carbs and the air box if you still have it to start with. You are going to see a huge performance boost with all this stuff anyway, and pods will make re-jetting more difficult. 29mm smoothbore carbs are probably an easy longer term fix. You could go bigger but there are lots of knowledge and parts for the old Mikuni 29mm carbs around and they work well with pod filters.
Steve darlington on the KZ\GPZ750 mailing list at
www.gpz.us has experience putting a 650 head on a 750 block. He should be able to give you a better idea of how that works. He also is a great source for parts and knowledge on these bikes. He's been vintage racing a kz650/750 for years.
Last, look for some cams to go in that motor. I'd love to see what an 810kit would do for my bike. I thought about this but decided to go the easy route and just did the 700cc kit. With the 700cc kit the cams made a huge difference. Much more RPM available and the power peak moved up to 10.5K.
If you want to get really wild, ARIAS has supposedly made a set of custom 880cc jugs and pistons for a drag bike. That would be expensive but way cool. I'd guess your looking at a few thousand to get this done. And I think you have to re-sleeve the block which will include more machining costs.
You might also want to start looking around for an oil cooler. With 80HP I've never seen an issue with overheating. I live in GA so that includes 100+ degree ambient temps. But I also run a rich mixture and only get about 28mpg. If you live in a hot climate an oil cooler would be a good thing with this type of motor.