Cb shop?

You can use a Bird Purch on the hole for the mirror on passenger side of the truck. 4 foot firestick with adjustable tip works best. Run the wire thru the door molding across the top of the windshield using cable clips that stick to the top of the windshield. Then to the center where the CB is. Also a Stryker 447 fits the factory hole too. One note, you need to use 18 foot of cable.

 

1.5 aint to bad. I run a bird purch with the firestick 4 foot adjustable tip with my 655 Stryker and I am at 1.0. I am able to pick up skip talk a lot when I am out west.

 

K40 has some premade coax with a smaller connector that lets you unscrew that big ol 259 connector.

Not the best coax, but it is better than stock. It's good enough to get the job done and try to split hairs for something better that you won't even notice a difference, in the real world, in performance.
Some truck stops carry it.
https://www.amazon.com/dp/B001JT5SAS/ref=cm_sw_r_cp_awdb_imm_t1_h1QuFbYH5NBMW

 

When I drove a T680 I used 5 foot firesticks. It was recommended by the CB shop that I mentioned above. With the T680 you need to use a top loaded antenna. It cuts down on the deflection from the mirror and the cab.

 

There is the best mounting solution for that truck. 81 inch antenna is ideal. Mirror needs disassembly to install cable. It's a long tedious job.

 

Bird Purch is so much easier to do than this.

 

here's a late suggestion...

Chuck's CB Shop in Holiday, TN. I-40, exit 126, go south a mile and he's on the right.

Both Woodchuck and his son know their stuff and will do you right.

 

This is the only mount you need for that truck.
https://driverextreme.com/dx368-kenworth-mount