I run panels I built out of 8 cree cxa 2540s @3500k with a typical cpu cooler style build and two hlg-120hC700B drivers mounted outside my 3'*3'*6' tent with a couple usb fans blowing around in there. Currently pulled down over 2g/watt using jack's 3-2-1 formula, silicates, liquid kept, and humic acid. Super cheap, calculated everything, 11 bucks a month electric including my one outtake fan, and three usb fans circulating air. Temps never get above 80F, really recommend using the cxa2540s, better bin on 3500K that the 3070s aren't even available in, way less expensive ($28/chip), and my koshertangies love em. Pro tip though, mount the drivers outside your growing space, heat is almost negligible at that point and your actual panels have a much smaller footprint.
400w HPS are alright, but if you're growing in an enclosed space, you're fighting a pitched battle with heat. While totally manageable, cooling the bulb can reduce efficiency and par output pretty significantly at up to 25%, stack that with a cooltube setup with a glass enclosure and there's another 10% radiant intensity through the glass. Cob led's are amazing, my panels cost ~$388 having had bus wire, soldering irons, etc prior to building them. Factor in how efficient they are (true 320w at the wall with ~80% efficiency), longevity, par output, heat management, and raw g/w, they're actually a cheaper alternative to HID lighting in a five year float. My tip would be to use definitely use a trellis and put your panels ~10 inches off your canopy, the heat won't come close to burning anything, the chips just have a native 115* beam angle that can be lacking in terms of penetration. There are reflectors available to sculpt the beam angle but they're an added expense and not entirely necessary with proper canopy management.
Driver:
http://www.trcelectronics.com/View/Mean-Well/HLG-120H-C700B.shtml
Chips:
http://www.mouser.com/ProductDetail/Cree-Inc/CXA2540-0000-000N00V235F/?qs=sGAEpiMZZMu4Prknbu83y1rZLMTsDUTv3oCGGoLiV2E=
Much easier build than OP's, I think. Just solder on a power cord to a driver, put the chips on cpu coolers with thermal paste, run a good amount of a decent gauge wire from the driver output, and wire the chips in series (no more than 5 per driver). I just superglued the cpu cooler's fan housing to a white piece of plywood, screwed a 12v molex supply to the bottom -> 8way 4-pin fan splitter, hooked the cpu coolers up, plug the 12v supply in separately, and plug the drivers into your timer. Boom, screw some hooks onnat biatch and rope ratchet away. And all this is modular, if you want a bigger panel, double the recipe. 1 driver = 160W, so I recommend two for a 4*4 situation. I'll upload some pics of my last wifi alien x gorilla glue run if I can find my camera.