Apologies
I missed the notification. My phone has been a PITA lately. Ordering a new one today.
16" would be a great distance between the edges of the heatsinks. It will distribute at an effective level. Setting the light in the angle works just fine. Just make sure the boards are not in contact with the frame (just the heatsink is). Also just make sure he doesn't have a really thick angle so the diodes aren't set back too far. An 1/8" angle should work.
as long as you box the heatsink in, you will not need a middle support. Each unit weighs about 10 pounds. You can use the four eye holes on the heatsink to fasten to the angle if needed.
Let me know if you have any more questions. Apologies again for missing the first one.