I had a 450-lb safe in the second floor of an apartment building, but that floor was designed to support the loads that come along in an inhabited space. Your attic wasn't. Beef it up like a real floor and you should be fine.
I wouldn't put that safe in the attic. Pretext: I've spent some years doing structural analysis and I generally tend to be a risk averse (ie professionally I can be a wuss) engineer at work. I highly recommend you NOT doing this for the reason patriot listed. I would say the only exception is if the spot in the attic is right up again a good load bearing wall. For example, the attic above my garage obviously has 4 walls. 3 of them are simply the outer garage walls. The wall up against the rest of the house is the main support wall for the upstairs.
Since you can see the beams:
What is the foot print of your safe?
Measure the spacing between the beams in your attic?
With the above 2, figure out how many beams you safe is likely to span in the position you envisioned placing it at. Remember, that plyboard is merely a covering. The beams are going to take and transfer to the load to the wall.
I assume your safe is wider than it is deeper. So choose an orientation that has the widest dimension of the safe span across as many beams as possible. Heck, get a piece of carboard cut to the exact foot print of you safe to layout the location that will distribute the weight over the most beams.