I HAVE to join this thread... I'm looking to upgrade with almost the same exact measurements (except a foot longer= 84"L).
Instead of the 4 MP40's on each side, I was thinking of 2 MP60's on each side (might be overkill or 72", but I felt it would probably work well with 84"L). I'll then have two MP40's on the back glass pointing towards the front so that the 36" front-to-back depth does not cause any dead spots. And then for good measure, I'm putting two Tunzes on a controller in the corners of the tank pointing diagonally inwards with my return lines taking care of the rest of the flow.
Having said that, I'm not planning on getting an overly powerful return pump, so I'll be interested to see how your choice works out.
Good luck!