I seriously doubt you have 300 HP. 240-250 at the Crank is more realistic.
2.73 gears are for top end speed. Pretty useless gears if you ask me. Go with 3.73's or 3.55's if you want more performance. When I had my 2.73's I did not have enough HP to max out my speed. I was limited by the drag on the car. 3.73's are a great all round gear and what I recommend you get.
The only experience I have with chips in the 4.6L cars is on Hammers GT. And you could definately tell you have a chip.
Take a picture of the engine compartment and post it.
The 96-98 GT's are a nice base, But I think they need boost to get them really singing
