I had a 387 for a year....I just ran the stock antenna's but my radio put out 65watts, so i probably sounded ok even with the stock antennas. I have seen guys run coax from the radio, into the bunk, out the side box, and under the rubber on the fairing and mount their Wilson or whatever antenna on the grab handle inside the fairing. You'd have to have a pretty big bracket/clip to go over them dang mirror brackets.
the 387 had two different ways, ether a mount above the windshield in the center or there was a kind of flat mounting plate on the dash, if no way above windshield, yours probably had the dash set up, just been removed, contact a pete dealer parts counter, give them your VIN, they should be able to tell you which set up had, good luck