The T5 retrokits have a single reflector for each bulb (called SLRs, these are essential for optimal T5 output). I think the Tek SLRs are about 3" wide, so you will need some type of convex mounting structure to get more than six bulbs on an 18" front to back tank. Since PAR diminishes quickly with distance, you might do just as well with 5 or 6 bulbs. On that note, the bulbs should be mounted as close to the water surface as you can without getting too much salt spray on the lights or color banding, I think 6" is a pretty common height. Also, cooling fans directed at the bulbs are an important part of a T5 setup. These will significantly increase bulb life and PAR output.

If you decide to go with MHs, check out the Lumenbright reflectors. There is a long thread on reefcentral about these. It sounds like they can be mounted fairly high and still give good PAR. You could probably get the same light coverage with two lumenbrights as you would with the 60" 80 watt T5s. If your tank has a centerbrace you could try this, but if it has the plastic bracing at two foot intervals you won't want to use just two halides.