The website I checked says that heater is submersible. The general rule is 2.5W of heater per gal
of water. Other factors to include though is how much heat your lighting will introduce as how much heat your pump might introduce.
I would go with a pair of 250W heaters. This would give you just under the 2.5W per gal
and if you you have MH
lighting then you will have heat comming off of those aswell to warm the water.
I would just lay them in the sump horizonal. No need for them to be veritcal or anything.