Over head is easy , even when tig and stick welding. Vertical up is another story tho and that seperates the welders and rod burners ( that is just my .02) . 9v mig welding should sound like bacon and eggs frying on the stove. Another way to test your welding is to do a t joint , weld just one side , and then break it apart using a hammer. From that you can tell penetration from the line of fusion and if your angle needs to change. The weld should break right down the middle if you have equal penetration on both plates, you'll see what i mean once you do this.
Plus welding t and lap joints keeps you in a straight line.
I know your welder is probably rated for 5/16 , but that is with flux core wire, which burns extremely hotter than solid mig wire. I know guys who have melted the plastic lens in their welding helmet welding with flux core. Also which gas are you using? Straight CO2 will make it burn hotter , than 75/25 mix.
Oh yeah,
practice ,practice, and practice some more to strive for the perfect weld.