I am not happy with my fender mount antenna. After reading the forum MMckenna has convinced me to drill.
You are going to be happier, it's going to work better, and it's going to look better.
I assume drilling on top of the roof rib is preferred vs the valley. The top of the rib is dead center of my roof.
Not sure which cab you have, but if it's one antenna, dead center. I agree about the waterproofing not making a difference. I've never had an NMO mount leak, ever. But then again, I'd not tempt fate and probably wouldn't put it down where water/snow/ice can collect. Given the choice I'd go on top of the rib, just for good measure.
Here's the roof of my personal 2018 F-350:
I know I need to use a quality NMO mount. Which model exactly should I get??
I've been using Larsen almost exclusively for 30 years now, never had one fail. Laird makes good stuff, too. Avoid Tram/Browning, any of the hobby/amateur grade brands.
Since I do a lot of this stuff, I've pretty much standardized on the Larsen NMOHF mounts. Overkill for 2 meter/70 centimeter use, but they'll work. They are rated up towards 6GHz, so useable for just about anything. They do cost a bit more than the non-HF mounts. I use them since I can have one mount for anything I need.
You can also use the standad NMOK mounts, they'll be fine for VHF and UHF work.
Note that the NMO-HF I linked to does not include the connector, so you'll need to supply a PL-259 and install it yourself. The NMOK mount I linked to does include a PL-259 plug, but it is NOT installed. Either way, that's good, cut the cable to length rather than coiling up excess. Much easier to install the cable without a connector on it.
Antennas, I have a 40" duel band on my fender 1/2 wave 2meter---- 5/8 wave70c. This seems to long for a roof. What duel band antenna would you suggest?
The Larsen Dual Band antennas will work on top of a full sized truck. My brother in law has one of those on top of his full size Chevy, no issues. If you want something shorter, you have a couple of good options:
This one is about 20" tall:
Or, you could use a simple 1/4 wave whip. It'll be 3/4 wave on 70 centimeters and will give you low SWR on both bands, and very wide bandwidth on VHF (good if you are listening to public safety stuff on your ham radio). I found the 1/4 wave VHF antennas work very well for my applications. The radiation pattern on 70cm is a bit funky and launches a bit high if you do this, but I rarely used 70cm and didn't have an issue with it.