which vortec motor is it, if it is 96 and up then they came from the factory with 260hp, but they also had a "vortec" 350 in 93-95 that used the swirl port heads instead of the 906/062 vortecs that are so coveted. these were rated around 220-230hp.
if you do have the 96' and up vortec motor then with some headers and free flowing exhuast and cold air intake you could probably pick up around 15 maybe 20hp. so it could be around 275-280hp.
like what Fbird said, the only real way to know would either be to run it on a dyno or to run at the track and use one of the ET to wieght calculators