Noticed those little rails the mounting plate sits in? there are three sets, on the bottom and on each side, you can use these and mount it on its side no trouble. I've tried with my HPPS (which is identical to the aquastream on the pum side) and it works fine. What I haven't tried is outbound-down, inbound-up and inbound-down. By the construction of the pump (and by deduction from various other eheim pumps which are similar in construction) those should be fine too, although you might have trouble getting all the air out. But I haven't tried it, so take it with a grain of salt, please.

You can also run the pump on the side. I also use it this way in my own system