A picture is worth a thousand words.
Looks like you're on the right track.
I would put the powerhead in the left rear corner as low as possible without sucking up sand/gravel. I would aim the inlet to the right rear. You can always change this later.
I wold locate the sump all the way to the right ( or directly above the drain location) to reduce the horizontal length of the drain. The added distance from the powerhead will have little or no impact on flow.
Careful consideration will have to be given to the drain size and vertical length.
Here is an excellent sticky that will help.
http://www.monsterfishkeepers.com/forums/showthread.php?t=205645
Hope this helps.