I would incorporate the following sighting rules for sighting terrain for 5 mile hexes. This based on the calculation of the horizon distance.
Clear terrain (Standing) = the hex in which the party is standing
Clear terrain (40 ft tree) = the terrain in the six surrounding hexes.
Hilly Terrain (assume 200 ft difference in elevation)
Hilly to Clear = Any clear hex within a three hex radius with no intervening mountain or hill hex.
Hilly to Hilly the same as clear.
Mountain Terrain (assume 1000 ft difference in elevation)
Mountain to Clear = Any clear hex within a seven hex radius with with no intervening mountain hex.
Mountain to Hill = Any clear hex within a six hex radius with with no intervening mountain hex.
Mountain to Mountain = the Same as Clear (tree) This is assuming in the hour of trying to sight terrain that a crag or summit is used to gain 40 ft of elevation.
Note that this the reverse is true, that standing in a clear hex one can see a mountain up to seven hexes away.
Trying to sight terrain would be a task taking about an hour probably cause at least one wandering monster check as the party roams the hex looking for vantage spots.
To me the Old School Renaissance is not about playing a particular set of rules in a particular way, the dungeon crawl. It is about going back to the roots of our hobby and seeing what we could do differently. What avenues were not explored because of the commercial and personal interests of the game designers of the time.