If it's the (yuky) 2 piece affair, trying oulling the two ends together first with some multigrips, while your doing that, have someone else bang on the back of the tank to get it seated in, then if you can, put the pinch set screw (bolt) in (these are usually 2" long).
If it's the one piece job (

), bang on the back of the tank with your hand to seat it, then put the set screw in, sometimes these are a little tricky to get started, but if you bend the end of the strap a bit, that should help line it up.
Also, a lick of rubber grease on the fille neck helps too.......