Here is the original Greyhawk 2024 map, the region of Arn.
Here is how the map of Arneson overlaps.
(This Blackmoor map by Bunce uses about the same scale of distance as the Greyhawk map. Likewise, the original Blackmoor map of 1971 by Arneson also uses about this scale.)
Here is how I modify the coastlines of the Greyhawk 2024 map to match the Arneson map.
(Also notice the subtle swampy Firefrost Channel at the location of the "n" in "Arn". It goes into and thru the Black Ice.)
Here is the location of Blackmoor.
Compare how the location of Blackmoor on the west coast of the peninsula is the same on both maps.
Here are a few more towns to get a sense of where the towns are on the Arneson map.
(Notice the Ruin is near Bramwald, on the west side. Maus is on the north tip of the peninsula. The distant Southport is here to help players triangulate the rest of the towns on the Arneson map. The locations are precise. The lair of the Egg of Coot is northwest from Blackmoor. Dantredun is a town on the original Greyhawk map. It exists just beyond the edge of the Arneson map.)