I think 60" is probably a little on the tall side for the L/R... 50" will probably be okay. I'll definitely be using a crossover from 60-80hz... probably 80hz. What kind of extension would be there with the 50" height?
I don't have a problem with the array... whatever will sound best. What would be the crossover of the midrange/tweeter?
If you can get the tweeter on the center speaker on the top half of the speaker when the speaker is placed horizontally without it effecting anything negatively, that would be good. That cabinet is a little lower than ear level when seated.
Like you did with this one...
How would two tweeters stacked on top of each other in the center speaker work out?