Just about ALL the gas is in wing tanks, in the mossie!
Also, there's no reason to get rid of the 300lbs of .303s, because that's barely even ONE percent of the loaded weight. You save just as much weight by burning off 50 gallons of fuel. Consider that it holds something like 550+ gallons total. 50% gives you some 275 gallons give or take and nets about 35 mins OFF wep (far less using WEP, this thing is thirsty) so you'll burn through FAR more than just a measely 300 lbs before, during, and after combat. 50 gals of gas is like 4 minutes off-wep. Burn wep for 2 minutes and you've already lost more weight than the .303 ammo.
At some point, nitpicking every last ounce seems to be futile. It's better to keep the .303s and empty them into the enemy. You only benefit from using them instead of wasting them. This also applies to some other very heavy planes. Don't dump tail gun ammo on 110s. Don't dump 7mms on Fw190A, etc. Better to just keep them and focus on flying rather than on less than 1% of the weight of your plane.
My $0.02.