Ice cap ballast & URI with the built in reflectors worked great on my old tank! It was a retro, I added a reflector but don't think it helped that much! Also on a side note you can use the same ballast & get T5 end caps & reflectors for the cost of the reflectors more, a very good choice also & they allow more room!
I’ll agree also with the ice cap and uri’s.
You could probably get away with using the 430 ballast since you plan to use only 24†bulbs. Although there’s only like a 20 dollar difference between that and the 660 and I’m not sure if what scooty said about the t5’s would still apply with a 430.
Agreed with the icecap ballast and URI's....if you are just adding 2 24" bulbs, you can save a little money by getting the 430 ballast instead of the 660.