800mhz phantom replace stock antenna

Hey, So I'm looking to get a mobile antenna installed on my car. I currently use a pro-106 with a 1/2 wave 800mhz laird rubber duck. It gets ok reception but i tested out a mobile 800mhz motorola 3db antenna on the front fender of my car(VW 08 Rabbit-same body as GTI) and it pulled in signals more clearly. I'm wanting to replace my stock AM/FM antenna with a laird phantom 800mhz antenna, as it'll look the best, and that's what I'm going for. I use sirius radio that uses a different antenna so the AM/FM is not needed. Here's the catch, where I'm looking for assistance. I want the stock hole to stay the size it is, which is about 1/2in by 1/2in square(http://ift.tt/1BHpgj8). My current 3/4 nmo permanent mount cant fit this. Would a 3/8 nmo fit without too many problems? I know there will be space around the center pin, but the pressure after screwing the nmo nut in place should keep it tight enough to not move, right?



Also, do you think the phantom should get the same results as the motorola antenna? The phantom will be on the roof vs the fender where i tested out the motorola one.



If this route wont work, my only other options i want to do are mag mount a phantom on my metal spoiler area(its not a spoiler, just part of top hatchback), or window mount a 800mhz antenna, but i worry my tint would hinder this as idk if its stock or not.

800mhz phantom replace stock antenna