The ISC 8.3 you can upgrade by a software change, however, the older mechanical 8.3 must be mechanically altered. There are fuel plates available from several manufacturers that increase the fueling of the mechanical 8.3c engines. Banks also has an upgrade kit I believe to squeeze some more power out of the 8.3 Cummins. I think it puts it up to something like 400 hp and 1150 ft/lbs of torque but don't quote me on the rating.
Mike.