Brendan B
If you are just removing the three bolts that go thru the tank mount, I would reuse the head gasket if the engine has not been run. You will need to retorque all the head bolts after it has been run enough to get the engine up to operating temperature anyway. Let it cool down before you retorque the bolts.
If the cross brace is still intact on the hood of your 73, the tank and mount from a 10 hp is probably all that will work, without the cross brace hitting the tank, unless you modify the mount and use the 7 hp tank. The 12 hp tank and mount is bigger, and the crossbrace will hit it when you try to close the hood, at least it will on a 70/100 and the 73 should be similar.