The 603 IP still uses a computer-controlled idle, so you would have to do something. Still, that's probably a lot easier than making a 606 IP work.

To use the 606 computer, you also need a few other components from the donor. Everything related to DAS, of course, and they all must match; you can't mix the parts even from identical cars. For a later 606 computer, you'll need the electronic ignition switch. Etc, etc, etc.