bulb covers an area about 2'x2'. If you put one on the tank, you'll have areas on each end that are dimmer. There is nothing wrong with that as long as you know that's what you'll have. Some folks use 1 MH
on tanks that size and put low light inverts such as mushrooms in the dimmer areas. I don't think 175w bulbs will really give you the penetration you need for a tank that depth...250 at least.
As for the actinics, if you use PC
's, I'd go with 96w bulbs...they're about 35" long. One thing you might want to consider is T-5's though. Coral Vue has a T-5/MH
retro kit available with 2 T-5 actinic tubes and either one or two MH
bulbs in your choice of wattage. It comes with ballasts, bulbs, power cord...pretty much plug and play. All ballasts are electronic as well. Something to consider anyway.