Most street lights around here are 150 hps. You could just take the guts out of the fixture and make your own box for your ballast. Just make sure you buy the light from the city sense they replace them. You can buy a 400 for $150 off ebay.
Sounds like alot of hassle. Wouldn't it be easier just to go to home depot or htg supply.com? I understand the thrill of the hunt when "aquiring" such items, but it seems pretty involving. Besides, from the reading I've done in the Jorge Cervantes book, street, security and wherehouse lights are among the worst in terms of spectrum. If it is a LPS light, it is described as "monochromatic", producing a very small portion of the light spectrum, around 589 nm. If it is a Halide, it may work for vegging, but you'll need something with a little more red in it for flowering. My $.02, save up and get a conversion bulb or convertable balast and 2 Hortilux bulbs, it will be well worth it.