Two CXB3590, 21" or 18" heatsink??

A build with HLG 185h c1400 and four (4)cxb3590, and two(2) 5.88"w heatsinks (front & rear, identical & attached), to achieve passive cooling, for length would I need 18" or would I be safer with 21"+?

3x2 grow area. Flower tent

Read this thread, especially the first post.

Well I did read that but I've already pretty much settled on 5.88"w, so I'm just trying to figure out the length

2 3590s @ 1400ma require 20" of 5.88@ 120 cm^2/heat watt

@BOBBY_G thanks a lot, I will order 21" to be safe I'm so excited about this build guys!

I'm at home depot now, can someone tell me the best size screws to get & what I need to attach and hang the lights by the heatsinks?

i drilled and tapped mine for #10-24 eyebolts and used these with acorn nuts on the bottom to tighthen and prevent spinning

http://www.mcmaster.com/#9489T47