Not much room left up there, but all you can do is place the antenna as far away from any mass of metal as practical.
Yes, you want just the metal roof below it for at least a 1/4 wave or 19 inches. So, 19 inches from an edge of the roof and at least 19 inches from any piece of metal that sticks up like fan, awning, etc. You want a good ground plane below it (the metal roof) that makes up the other half of the antenna system. You want nothing near the part of the antenna that sticks up that can detune it. If you offset it to one side of the roof, it will usually favor the direction with the most metal ground plane.
You really should get the antenna without the connector. It is much easier to fish the coax through where it goes and then put the connector on. Someone should be able to help with that. Installing connectors is a skill you should have. I make up most of my own cables. That way they are exactly the length you need. I think Larson sells kits with the connector separate. I drill a pilot hole and then use a Greenlee punch to make round holes. You can get an endoscope to help fish wires.
Temporarily, a mag mount will work well too. I easily hit a mountaintop repeater 100 miles away. HAAT (Height Above Average Terrain) is your friend. A 6000 ft mountain helps a lot, but a 9 foot Sprinter vs a 5 foot car is a help too. Look at this calculator:
This radar horizon and target visibility calculator determines the geometric target visibility and radar target visibility (taking into account the refraction ...
www.translatorscafe.com
Put in your height for one (radar but it does not really matter) and the other end height for the other (target). If you have two antennas at 9 feet instead of 5 feet, you get a couple extra miles range. A 6000 foot mountain and a 9 foot Sprinter will easily get 100 miles.
Regards,
Mark
