you could do 10 if you wanted. Jus gotto keep em very low and short, flower them very very young, Just remember the intensity drastically decreases with space from the bulb. I heard for 150w anything 13 inches away is useless. For a 70 watt id imagine 6-10 inches is the farthest you want to get before you start heading down fluffy bud road.
But there is still hope, shop lights are cheap walmart ($15 after you add in the cost for the bulbs) red spectrum and blue mix them up half and half stand them up vertical to accommodate your space.
Now that you have a local place to get these lights at a great price here is what you will end up with:
3 shop lights banks holding 6 bulbs at 40watts a piece not equivalents!
Thats 240W (real watts) of cool energy efficient florescent tubes now you have 310 Watts of flower power
Throw your HPS on top of the space and you can get by.....
But in the meantime Dude veg your plants as long as you can for size nothing but shop lights grew what you see 1 light at fist until they out grew it then I added 2.
i asked if i could use a 70w hps and a few people said it wasent enough so i got a 250w instead.this is my first grow and i like as much weed as possible cause i'm sick of buying shit from idiots so if i kept my plants small the weed i got of them wouldnt b enough to keep me going for 12-14 weeks.i've still got the 70w but my cuboard isent really big enough so i was gunna make a box.i've got loads of ideas and info of this site.cheers