You might consider putting the fillets on the outside, to allow the side stand clearance. Not a big job and looks fine in my opinion. As pschrauber mentions, the plastic tank is much better than the metal one, the staining you see is not as much a problem in uk weather, but emptying the tank before storage should stop it happening.
Fitting a metal tank requires welding two bosses near the headstock, and find suitable rubber bungs to locate the tank, also you will prebably need a seat to suit the tank, as I think they are different fixings.
Bike looks very nice btw. Check the kick start is tight as you will knacker the splines, and they do come loose. When you change the gear/clutch oil do not undo the 17mm bolt, its the alan key one to drain.