Are you planning to use fiberglass? The bottom of that enclosure you posted in the first post is just metal. That might resonate a bit too much. People generally lay the bottom and sides using fiberglass, then make a MDF top.

Anyway, 300 watts RMS and a smaller 4 ch amp is not that power hungry. If your 92 Si's electrical system is in good condition, you probably can keep the stock alternator. If you want, you can find a 1 farad cap to bandaid the issue. But if your alternator is about to be replaced anyway, going with a higher output alternator is the way to go. But I wouldnt unless I was planning to use all that juice.