Thank you. What do you mean by 800A tap box? Also, problem is whether it's bus or wires from genset to breaker.Junction box and polaris type connectors? You'd most likely need to upsize from #10 to fit the connectors spec.
Or you might find an 800A 'tap box' that works.
There's more than one way to do it.
It's easier to ask here first even when the answers are in plain sightLooking again at your drawing it describes everything as 'existing' so why aren't you getting your answers from the site?
I'm sure you would need to supply more information, such as make and age of generator, for anyone to guess what's there for you.