I would guess 250-270 rwhp for that combo. There were no Irocs with 260hp. The highest power rating in an Iroc car was the 90-92 Speed density cars at 245 hp. But, they also made 345 ft-lbs. This is flywheel numbers anyway, so probably around 200rwhp, which is a lot less than your combo would make.
With some sticky tires, a good convertor and some shortie headers and you'll have a nice little sleeper. You shouldn't have a problem with the IROC either.