I hang mine at a 25cm distance, but i have 4 @ 1400mA for 75x75cm. And when i have them closer i get some slight light burn.But you can try at 15-20, and if you get burn you can raise them higher. For passive cooling you need ALOT bigger heatsinks, and if you do not have a good place to get them cheap, i think the cpu coolers and a 12v psu will be cheaper. The heat around the heatsinks will not move unless you have really beefy ventilation, and therefore you need a bigger surface on the heatsinks. I found a 12v1A psu at ebay that cost something like 3USD with free shipping, and the Artic Alpine 11plus are sold for approx 10 USD a piece. The efficiency might not be great, but you are running 2x 0.16A fans from it, so total energy consumption will be low.
Could I use the 12V1A psu to wire two PC fans each running 250ma in series, or will the voltage drop to 6V. I get confussed about if they need to be wired to parallel or series. I would use it to provide ventilation in the grow room. As to the passive heatsink I will be using NOFAN 85 and have tested it running the 3070 at 700ma and a small draft seem all that is needed. I was lucky in getting 6 of them for free. The only down side was their height.
So in summary in the flower room I will have
2 x CXA3070
2 x Meanwell APC-35-700
2 x NOFAN passive heatsink
2 x 12V@250ma 120mm PC fans
2 x APC-12-350 to drive fans ( mayby replace with your suggestion of the PSU )
1 x 12V 100mm extraction fan with Rhine filter.
And the veg room
1 x CXA3070
1 x Meanwell APC-12-350
1 x NOFAN passive heatsink
1 x 12V@250ma 120mm PC fans
1 x APC-12-350 to drive the fan
Every thing will be controlled and timed via a programmable extension socket Energenie four socket power management system.
That in a nutshell is everything minus the grow room's which will be made from 3 layer sound proof clading incased in a slightly larger box to run silently at around 15Db.