if you keep the volume low enough to where the cab doesn't blow, then you will be fine. but if it were me, i'd only run it through one side just to be on the safe side. and adding a 115 wouldn't be so good...the 410hlf will bury it in volume and low end. if you ever want to add a second cab to the hlf, a second 410hlf is your best bet.
Like Jimmy said, it's fine to run more wattage than your cab is rated for, you just have to be careful to listen for when the cab starts to distort and turn it down if it does.
also +1 on the second 410hlf - if your current one is 8ohms, get another and run bridged into both and warn the neighbourhood before the earth tremrs start...
my 410HLF is 4ohms, as far as equipment size goes I simply can't have another one altho that would be earth shattering , I'll continue to run through one for gigs but pay close attention to the sound, thanks guys