As to which units are overdriven, you'd have to ask the specific mfgr and or sales source. Aquactinics' Solar Flare is overdriven producing about 80 watts per bulb vs. the standard 57-60w. Contact Tom via the sponsors forum.
Ballasts can be replaced pretty easily. Icecap ballasts generally overdrive the T5 lamps safely. You'd have to check the icecap forum in the sponsors area to find out exactly for your application.
What I do know is that an Icecap 660 will overdrive 4ea 48" T5 bulbs and a 440 will overdrive 2ea 48" bulbs. My aquactinics fixture utilizes these ballasts to power the 6 bulb unit I have.
What are the deminsions of your 60?
Check this site for retro's
www.reefgeek.com and this site for premade hoods
www.aquariumspecialty.com Greg at reefgeek can give you more info. The aq-specialty site is not the cheapest but by far the best customer service of any that I have delt with in the past. Call Scott, he'll help you out with information on their aquactinics units and or you can check out the
www.aquactinics.com website and look in the sponsors forum (they are listed there too). The owner of aquactinics name is Tom and he was very helpful when I was making a decision on T5 lighting.
Sorry I was repetitive in the answer but you should be able to get some direction and info from searching around in the places and people listed.
HTH