[chronomafia]

I know that the blocks and all of that are the same and with the engine being a vortec you will be getting better air flow and all that due to the porting on the heads. The only thing that I can think that could be the issue is the distributor? The Iroc used a vacuum advance set up right?
Also, some of the sensors may be different as well as the motor mounts. Tranny will bolt right up no problem though. I would use everything thing that I can and just swap the intake and distributor from your old motor.