Hello 70Cutless442: Yeah the Hy-Gain SPT-500 will work well. I would NOT go thru all the effort to put up a A99 on a tower.
Yea it will probably have a slight pattern distortion mounted on a tower, but only a slight one. I have assisted in installing a couple of these antennas side mounted on a towers at hundreds of feet, and they work great.
Hy-Gain
I wouldn't loose any sleep over the 98.408 none sense.
Using the Heliax Coax for a two to three hundred feet run is also a good call. But I have used LMR400 for a 350 Ft run once, it had approx. 2 to 3 dB insertion loss, but the incredible height made up for it all.
Good luck, please let us now how you made out there....
Jay in the Mojave