Here are the answers to your questions:
1) The 4.25" x 23" heatsink would be a good fit for a 29g tank (I'm assuming it's basically 30" x 12" x 18"). If you're running 24 LEDs then fans are definitely suggested, ideally you'd have two 60mm fans (on on either end) but you could probably get away with just one fan in the center (in that case I'd probably go with the 80mm fan).
2) Dimming accessories are not included, so you would need a couple of pots + 10V AC adapter or something like our 2 color dimming kit.
3) Yes, that is plenty of light for SPS/clams in that tank. For the clam, it depends in part also on which lenses you'll use (I'd go with an 80 degree lens, but only if you can get the lights more than 8-9" off the water to avoid spotlighting).
I was looking at purchasing your 24 dimmable DIY kit. I have a couple of question regarding this.
I would be putting this over a standard 29 gal. I would like to grow SPS at least in the top part of the tank.
1. What heat sink would work best? 4.25X23? or something smaller? Would I need a fan on the 4.25X23 heatsink, or does it not get that hot?
2. Do I need to purchase the potentiometers for this kit, or are they included?
3. Is the 24 Kit enough light to grow SPS? What abut a clam in the bottom of the tank?