Don't need another tank to weld, epoxy, drill, cut or splice if ya are lookin for a sight glass.
LIke Perry said, but need to vent top to tank.
Just tap into the fuel line close to the petc0ck w/ a tee, buy a used gas cap, and run a small diameter clear gas-proof hose back up to your cap and install fitting in the guineia pig cap to vent back to tank. When the petc0ck is opened the level will match fuel in the tank.
Up to you how/where to route the hose, Maybe right down the middle & under the seat??
Don't like it?, just run another hose from the petc0ck back to the carb like before.