1. Poor grounding is the #1 cause of high SWR readings. The electrical bond between the antenna bracket and body is critical, as is the bond between the antenna bracket and antenna mount. Although not absolutely essential, I would also recommend that you use a piece of 1/2"-1" braided copper strap between body and frame and another from exhaust pipe to frame.
This article on bonding is quite helpful:
http://www.k0bg.com/bonding.html
You can buy copper strap here:
https://www.amazon.com/dp/B003HGHQYM/?tag=wranglerorg-20
and here:
https://www.amazon.com/dp/B01BIBQ940/?tag=wranglerorg-20 .
Just cut it to length and crimp ring terminals on each end.
2. Your power source won't affect SWR.
3. Loosely zip tie excess coax in a figure 8 or skein, never a coil.
4. Your stud mount is assembled correctly. The nylon insulator goes on top of the mounting surface for a Firestik K4A stud mount that uses a SO-239 connector, as illustrated on the left in Post #2.
Don't be confused by the two Delrin isolators I use for my Breedlove stud mount which employs ring terminals rather than a SO-239 connector and is akin to the center illustration for the Firestik K4.