I can only help on a few points.
Most I've seen go horizontal (the long way..or side to side...not front to back.)
How are the bulbs different? Color temp or what?
They should be mounted in the canopy about an inch from the wood, you can buy metal spacers at your hardware store (I got them at ace for about $.45/piece) and just ude longer screws. As to mounting the sockets in the reflectors...not too sure about that, my reflector had the sockets riveted in by the supplier.
If you can keep the PC
, you could use them for Actinic if you go 10,000K on the halides. What size tank is it? Hope this helps, and anyone feel free to correct me.
125 gallon saltwater: 55gal sump, 4 Blue-Green Chromis, Purple Tang, True Perc, Firefish, Royal Gramma, 5 Ricordea, Bubble Coral, 15 Pulsing Xenia, Green Star Polyps, Deresa clam, Green-Tip Torch Coral (about 11 "heads"!), Orange Montipora Cap, Purple M. Digitata, Green Slimer Acro. Yongei, 3 Orange M. Digitata, Pink&Green Acro. Millepora