I'll take a guess that your ballasts won't work as expected on a shorter tube. Zink will know for sure.

My advice, as it usually is, is to discard crappy old fixtures and get something designed within the last five years. You'll get more light, save a bundle on your utility bill, and your plants will thank you. When you start looking for something shorter than 48", look at compact fluorescents.

You can mount your components on anything you want, the temperature of any individual piece shouldn't be hot enough to harm wood unless you start overdriving. As an extra safety step, prevent any components from actually touching the wood with suitable spacers or mounting blocks.