The first time I took off my tank and tried to put it back on, it was a b!tch to get the two front mounts to line up.
My only suggestion is to have someone press down and forward on it while you wiggle the mounting bolt towards the hold. Turn the bolt with your hand or a socket extension only, not with the wrench itself (you can strip bolts really easily like this, but you won't have the strength to do it without a lever)
If that doesn't work, WD-40 the gas tank gasket as well as you can to make sure it slips all the way down on it.