What kind of radar? Is it integrated with your chartplotter / sonar / multifunction or a standalone unit? There area lot of considerations, so more details will be very helpful.
I'm not the leading expert by any means, but if you already have a display then you might consider getting a Garmin antenna unit so that it plugs into your existing hardware. Cheaper too, I'll bet.
you can mount directly to the hardtop or install a mount on the hardtop to get it up higher. I'd research it on the internet. You want to get the angle correct so you view the area. Too high and you miss what is in front of you and too low and you miss things further out.