Your setup is very similar to the 93 Cobra if i'm not mistaken. They were rated at 245 at the flywheel, and they were underrated, so lets say 230-250 at the wheels, plus your exhaust and other little bolt ons, maybe 270ish at the wheels. 300 rear wheel horsepower is about 380 at the flywheel. I think you will need a little more than your setup to see 300 at the wheels.
I'm not trying to be a prick here, but I think you are overestimating this. But, there are only two ways to know, run it or dyno it. So we'll have to wait and see.
I hope the upgrades make a huge difference. It really all comes down to how much fun it is to drive, and I'm sure you're gonna love it.
btw- Who is your mechanic? I had work done in Taunton at Leonard's and let's just say I didn't have very good luck.
Good luck
-Mike