Problem with the 180gal is its what...72" long? A typical MH will spread or cover about 2ft of tank space. So at 6 feet to get full coverage without many shadows you would need at least 3 fixtures. 2 would work, you can always build your structure around the lights, but just saying for a fully lit tank 3-4 fixtures would be ideal for a 180.
I'd probably end up going 250 on a 180 as well, you're adding about 8" of depth to the tank from a standard 40gal to a 180gal.
Could you get away with the 150s on a 180? Possibly, but you'd have to aquascape accordingly and basically keep all of your SPS and clams high in the tank. You just have more play with 250s at that depth.