a slip sleeve will kill your SWR's, and even with a 102" whip, you still kill your ground plane. As long as your not talking DX and don't mind a 3-4 mile recieve, it will be all good. The best placement on these rigs is dead center on the roof, where you are now. That way, it uses the roof for a ground plane.
I'm pretty sure he mounted that on his factory cargo bar, which has rubber ends and no ground, similer to the setups on KW t-2000's
He needs ground.
I run a Texas Ranger TR-296 (old school) with a 12 watt dead key, swinging around 27 watts. I run RG gold coax 18' hooked to a K-40 mag mount. my SWR's are flat on 1 and 1.3 on 40, and I can recieve 7-10 miles with the squelch on.
When I first installed it, I could barely hear the flying J 1/4 mile from me, so I started researching stuff till I learned. Beleive it or not, everything I learned on channel 19 trucking about CB's was wrong. LOL