I will start a build thread as soon as I start to build it
For heatsinks, I found some used ones on eBay. I bought three that were 12" by 15" in size so that will give me good coverage over 48" x 18". For your tank you will probably need something around 12 to 13 inches wide and 28 to 30 inches long.
And as Steve said, the number and spacing of LEDs along with what optics to use will depend on what your stocking plans are. I am using 4 rows spaced 3 inches apart front to back and then the LED spacing along each row will be 2.25" with 60 degree optics. That should give me similar or slightly better performance to a set up using 2 x 250W MH. You could go with 40 degree optics but then your spacing needs to be no further apart then 2" between LEDs in the rows and that would probably give you PAR equivalent to a 400W MH. Probably overkill in an 18" deep tank. I may use 80 degree optics in a few areas or set it up so i can dim it by sectors to have some lower PAR sections for LPS etc.
If I were setting one up for a reef in a tank your size I would probably do 4 rows of 12 or 13 emitters with 60 degree optics.