Where your antenna location now is a good as your going to get. Your correct, it is the metal underneath an antenna that acts like a ground plane effect, not the metal next to it. Omni directional is best for a mobile.
Like all antennas, get it up in the air and away from being next to metal except metal underneath.
A full size 1/4 wave whip is best if you want the most out of your mobile radio, but the gains will be minimal compared to what you have now which seems to be a good antenna and ideally mounted.
1/4 whips generally have more bandwidth and are usually plug and play on 11 meters with no tuning required. At best the DB gain over a good coil loaded antenna is less than a 1/4 of a DB difference which no one on the other end will notice. It takes a 6 DB increase to gain one signal strength on someones else's radio meter. In wattage equivalent, you would have to increase your power 4 times to equal a 6 DB gain. Even a 1/4 wave whip has some current losses since they are not 50 ohms at the antenna feed point so a 1/4 wave whip isn't perfect either.
If you still prefer to use a 1/4 wave antenna then consider this. Your truck is a commercial vehicle in many states but not the type that I see required to pull in weigh stations where a DOT man could cite you for being over height. Also many states have height regulations that are at least 14 ft. You may want to look your state regulations on this to find out for sure.
Since you have welding skills, I would make up a bracket or maybe even use a 1 inch diameter galvanized pipe on a threaded floor flange which can bolt down to your truck bed for a mast to rise up from behind the cab some at least to the rear window level, then you can attach an antenna on the mast with a CB mirror bracket and adjust the height.
