Rene Tower #3 would be one l o n g run of coax, then add the vertical distance. I was told they strapped it on the closest stick past midway to keep the run short. I'm thinking the effeciency loss would be terrible over that distance in those days. Plus, it would have been two bays vertical and two horizontal polarization, there wasn't Circular Polarization as today. I also recall it being a three to one power ratio, three horizontal to one vertical. Since most all FM listening was at home or office, the standard antenna was a horizontal dipole or TV antenna (also horizontal) split off to feed the hi-fi. There was so few FM car radios (Cadillac, Imperial and Lincoln) there was no need to waste power for their vertical antennas. And most any FM portable came with a swivel antenna just for that purpose, to pick up the station whether it was verticallly or horizontally polarized.
I think the power out at 18 k is generous. I would think 10k out at 70% efficiency, then into those dreadful antennas...It would be lucky to get 10-12K (3 bays) horizontal and 4k (1 bay)vertical. Although close to your 18K, those figures are the power which the radio antenna saw, and far different from today's 18K equivalent.
Other than a quick drive-by, I haven't been at the plant since '77, so I'm sure I'm forgetting quite a bit. But by today's standards, it was basically a glorified Class A.