What's the proper distance to hang barebulb 600s side by side?

I am building a light tree with a main vertical bar and 3 sets of arms (each arm will hold 2 600s at opposite ends)
Any advice on vertical spacing and horizontal spacing?
Thanks in advance for any help!
Bless

I don't think it matters as long as it's cool enough. It's the heat you're worried about.

I'd say 4-5.5ft, when you're hanging parallel lights there is an effect between the lights called the third light effect because the area is getting light from both lights and its as if you have a third light in the middle

Do you think 38" apart is too close side by side?

no, try it and if u find its too hot just throw in a floor fan in the middle