I believe you can use 2 rows of 6 Cree XM-L LEDs, with the LEDs 3.5 inches apart, and the rows about 6 inches apart, with 60 degree optics, and get about 70-80 micromols of PAR at 1300 mA current, with the LEDs about 20 inches from the substrate.
That would be high light, or very close to it. With the dimmer it would be easy to adjust that down to a more reasonable 40-50 micromols. If you use 7 LEDs per row, spaced about 3 inches apart you would be almost certain to get at least 80 micromols at less than 1300 mA current. A single Meanwell ELN60-48D should be able to handle that load, especially if you adjust the voltage a bit higher, and limit the max current to maybe 1200 mA.