That sounded pretty good as my checking showed that the gland nuts I have would pass the 3/8" FME connector, and they'll tighten down nicely on the ~1/4" thick RG58 coax. However, when I went to the Midland website it cannot find an MXTA51. And when I Google that number there are no results on the whole of the internet. So I have no idea on price and no way to order it.
I sent Midland back an email and turned to looking for the SO-239 connector to mount in the back of the cab. I decided I wanted a "rear mount" connector, meaning one that would slide through the hole in the roof from inside the cab and be secured with a nut on the outside. That's because to have a nut on the inside would mean I'd have to either drop the headliner or remove the cargo light to get a wrench in there, and I don't want to do either.
Called Digi-Key and they don't have any panel mounts that don't use 4 screws in the corners instead a nut. Called Mouser. And checked several others as well, including Amazon. But no one had what I was looking for.
Finally I found a page that give me a part number for the Amphenol connector and Googled that. Got a hit at Derf Electronics and they had the part. After finally getting to the right guy I got a price of $23 + shipping, so I bit the bullet and ordered it.
This solution is the one I've wanted with one minor issue - the Amphenol connector isn't waterproof. But it looks like the ones that are just have a rubber washer under the nut and washer, and I have one of those so I think it'll be fine. And I can use a bit of RTV as well.
But it will allow me to change out the antenna w/o going back into the headliner, so I think it is the right approach.
Sounds like a plan, bit of a headache, but should work great. I think you can seal it up also.