If I may speak to your questions;
The drivers look to be the inventronics 190v 1.05A, which are the same as the meanwell hlg-185h-c1050s I will be using. They are single output high voltage (somewhat more dangerous) but have the best efficiency offered.
If you want a safer voltage and plan to run around 1A, inventronics offers drivers with 2, 3, 4, 5, and 6 output rails (EUC series). The 6 rail is interesting because it has 38V ouput and is high wattage (some models in this range from inventronics only offer 36V, which is not quite enough for the cxa 3070 or vero 29). Of course you do lose a bit of efficiency with the lower drive voltage conversion. They are pricey though, at about 140$ for the 250W model.
Inventronics also offers some lower current drivers that would be excellent for running multiple strings of vero 10s without having to match strings. I believe when I was looking through the models I found one that could run 24 - 28 of them in strings of 6 or 7 off 4 rails. With the recent price drops the vero is very compelling given the variety of CCTs available (no additional red or blue required) and the ease of assembly (t slot or pre drilled heatsinks available) and the spread you can achieve (may matter more than the improved efficiency of the cree COBs).
I personally went with the Cree because my hydro is pricey and absolute efficiency matters to me. 3000K cree is still much better than HPS spectrum anyways, so I don't think we are losing much given the ease of assembly.
Hmm, well that was a bit of a ramble.... Back to your other questions.
The heatsink looks to be about 8" wide based on the driver, if I am recalling the size of my meanwells correctly.
I would imagine the black box will serve to house the AC connections as well as the 12V DC PSU for the fans.
Positivity, your light looks great! Can't wait to see it in action! I will be building using the same parts essentially, but will have 5 COBs per driver.
What are your thoughts on the reflectors? What height will you be hanging your light from the canopy? I was planning to go bare (but haven't 100% committing to my design).
I may have missed this but could you give a link for the driver please? What footprint is that massive chunk of heatsink? Also, what lives in the black box next to the driver on the top?
I love it - Fantastic effort with a result to match.