450watt HPS light???

The space i am growing in is 15ft long but only about 4'10 inches tall. I have a 1000watt HPS light and two 450watt HPS lights with 2 ballasts. One of the ballast i have, the light sits more into it than the other one and it is about 3 inches larger than the other one i have. Right now i have the larger one setup but can always switch, i was just wondering if the 450watt light would be the right light to use and considering the pot i am using and how big the ballast is i would only have about 17 inches from the light to the top of the plants?? is that good enough length with a fan on 24/7??? I have 7 plants that are about one month old, have 3 sets of leaves, and are about 3-4 inches tall, under four 40watt flourescent tube lights 24/7 since the day i started. When i move them under the 450watt HPS light can i switch the time to 18/6?? or will that effect the growth since i already started 24/7? can my plants be 17 inches or even 3 inches away from the light with a fan on and have a good yeild and such??

so wait im confused now, so you have those shop halides where the ballast sits on top of the reflector??? if thats the case i suggest you take the ballast off and move it out of the way so you can get more head room, its really easy to do and all you will need to buy is some cord to attach the 2