I did it on my SDL last summer, dropped quite a bit. 4-7C easily.

I also wasted plenty of time before finally replacing the radiator (due to a broken neck) and actually solving the problem.

Replacing the rad in mine dropped approx. 20C on average. And thats with a generic-brand radiator, not even a Behr one. The fin pattern is totally different.

I had no idea running pure tap water was a bad idea, I've had that in my system for about two weeks. I was meaning to change it out for 50/50 water/alum-safe antifreeze but just didn't get around to it. I guess I'll do it first thing tomorrow.

60/40 is what should be used.

Original radiator! Well thats your problem, lucky they are cheap for those cars! Well compared to an SDL radiator.

The fan has nothing to do with cooling at highway speed.
If your sure the thermostat is operating correctly( I have seen bad ones right out of the box) , your belts aren't slipping,your lower radiator hose isn't being sucked together(rare), then it's probably the radiator.