At this time I think 48 LEDs will be enough, in two rows of 24, with the rows stopping about 6 inches from each end of the tank. I think the rows could be about 6 inches apart and give you good uniform coverage of the substrate, with no spill over.
With most, if not all optics, the center half of the beam is much brighter than the outer half. At 36 inches, 60 degree optics would give a total beam width of about 36 inches, with the center part being about 18 inches in diameter. I would try to get that 18 inches end just about at the glass, with the rest of the bean being reflected off the glass to the substrate. At the top of the tank, the beams would be about 12 inches in diameter, big enough not to look disturbingly like spotlights. With the LEDs 6 inches apart, the beams would extend to 3 inches from the glass at the water line. That leaves only a small wedge of water at the top of the tank, front and back, with very low light.
40 degree optics would give beam diameters too small to look good, in my opinion.
Those drivers produce 1.3 amps from the factory. You could adjust the internal pot on each of them to reduce that to 1 amp, if you want to limit the maximum current. With the heatsink you are using, that is probably a good idea.