Stripping the paint off in the long run was probably a good idea, since Mother Nature has a way of doing it for you anyway. But I doubt the paint was a problem to begin with as for as your SWR go's.
Kales right about using just enough coax to get from point A to point B, as long as your just tuning one antenna. (however for every person that tells you this, you can find 2 more people that will tell you different!)
Even if you get Kale to send you a 27" lower shaft, having the antenna that close to the back of the cab, your going to have problems getting your SWR under 1.3....Just too close to the cab.
To prove my point, if you were to get a magnet mount and stick the antenna on the roof, even with the shorter shaft you would see a much lower SWR...Or move the antenna half way back on the bed rail (half way between the cab and tail gate) and you would see a improvement.
For as the type of coax your using, I have used RG 213 for years and never ran into a problem tuning...But if you want the very best coax, get some LMR-400........