super chip anyone?

Welcome to StangNet!
Personally, I have the SCT (Superchips Custom Tune) chip. These (chips or handheld tuners) are available via mailorder or dyno tuned. With mailorder, you can specify your mods (from mild to wild) and they'll give you a tune based on your car, these mods, and your personal choices. The benefit of a mailorder tune is price. A dyno tune will cost $200+ over the cost of the chip/hand held tuner.
Ck out ModularDepot---> http://www.modulardepot.com/index.php?cPath=21
300 HP possible without a power adder? Easy, if your talking gross HP, not RWHP. Just add full exhaust (headers, midpipe, catback), pulleys, K&N filter (or CAI with a K&N, but don't expect too much - and often these can create more problems than power) and maybe a plenum and 70mm TB and a nice tune and you'll have your 300 gross HP.
300 to the wheels would take more work - heads, cams, etc...
Enjoy!