I've got a 250 gal, I think the 300's are usually the same 8'x2' except they're 30" tall instead of 25", right? I've got an Oscar, a 5" Chocolate Cichlid, a 6" Green Terror & 6 3" Silver Dollars in mine with an FX5, 2 AquaClear 110's (500 gph) & a 1200 gph powerhead. The bottom is always clean, water quality is clear & always perfect. Your stocking would be similar to mine for the most part. I'd use the FX5 and grab a couple AquaClear110's for $34 each from this link:
http://www.drsfostersmith.com/product/prod_display.cfm?pcatid=3610&prodid=6173&catid=113
and I think you'd be good to go. I'd also cycle with straight ammonia before you add fish. I like understocked tanks. There's still plenty of activity in the tank yet room for each fish. It's still 'crowded' in their minds but they adapt a lot better than being crammed in an over-stocked tank.