i thnk your tank and my tank is about the same size, wats the height of your tank? i think most likely 1.5ft?
my tank is 2ft tall, so i place my MH really close to the water surface, almost like 4inches above only...if u wanna have a good spread, i suggest u change it to a 250W and take out all the PLs to save some electrical bills...
by placing the MH so high up, it will reduce the intensity of the light very much...
just my opinion, may not be the best, may even be a bad idea...
another thing, the nails and screws, use the glue tubes to seal them up..it is those that pple use with a glue gun...wat i did was soften the glue tube with a lighter and then smulge the soft glue onto the screw and nail head...this will help prevent rust...