Hello ALJ: YEAH! That's a good call.
I have always wanted to try two coil springs, did you modify the whip antenna length?
Mounting the antenna on the trucks tool box will effect the pattern some what, and effect the SWR, but your SWR is 1 to 1.0 as you say its possible that the antenna being semi close to the cab may even help tune the SWR. Would like to test that out sometime.
If you get the chance, listen in on a local station a few miles away, with a low S-Meter reading (S5) while driving the truck in a circle watching the S-Meter, to see if there is a distortion in the antenna pattern by being mounted close to the trucks cab.
I have made fiber glass whip antennas about 100 inches long, using large fiber glass tubing sections, that needed a heavy duty Spring. The fiber glass tubing had heavy silver braid inside.
I settled for a custom base loaded antenna using a 72 inch / 6 Foot whip antenna on top of my Trucks Cab, allowing good radio performance, and not hitting every low lying cable wire, tree's, and other things.
Good going.
Jay in the Mojave