Mounting CB radio overhead, How long can the screws be?

[Cadillac Kid]

Hello everyone.

Alright, it's not actually a CB radio but a 2 meter radio or "ham" radio. Sorry.

I have decided the best place to mount it is in the center of the headliner at the very front. I'm going to use some screws or self tappers to mount the bracket, but I don't want to accidentally drill holes through my outer roof.
How long can the screws be? I know these trucks have two layers of metal in the roof and I only want to go through one. The size of the radio is equivalent to a CB radio, it just weighs a little bit more.
For all of you hardcore CB radio fans, I grew up on them. I still use them today and I have one in my other car.

Any help is appreciated. Thank you!