You can actually replace the spark plugs without taking the tank off, although the service manual says to remove the tank. (I guess if you have super jumbo hands, that might be harder.) With the tank off, they might be easier to get to, but then you have to deal with removing the vaccum hose, the fuel hose, the fuel tank breather hose, the water drain hose, and the fuel level gauge wire. So just do it with the tank in place.
Following that logic, I would try to remove the petcock without taking the tank off. But taking the tank off is not hard at all.
You remove the screw that holds the petcock valve handle on (leave it in the "on" position). (You take this handle off only because the handle will not clear the frame when you pull the tank up.) You remove the bracket that the tank is attached to under the seat. The service manual would have you remove the hoses, but I have learned to put a 4x4 post under the tank to raise it enough to get to the air filter.
The "fuel valve" is held on with two screws. The OEM service manual says to replace two gaskets and and o-ring when you take the valve off for cleaning/replacement. I hope they came with the new valve you have.
When you put the tank back in place, just tuck the front back in place first, and the back end will line up where it attaches under the seat. Watch that you don't pinch any hoses. Last step is to put the petcock handle back on.
I have not replaced the fuel valve, but I would be OK doing it if I had to...and I don't do much beyond the type of things I've described above. Good luck!