I actually bought the O'riellys one but haven't installed it yet. Looks to be the same as the NAPA one.
Even if doing a 12 volt conversion, the advantage of keeping the relay is that it takes the switching load of the starter solenoid off the ignition switch, which is known to be a weak point in many older GM vehicles. With the relay, the ignition switch is only turning on the relay, which in turn handles the main load of the starter solenoid. Is it needed for a 12 volt conversion? No but it's a great thing to have in place.