imo a truck that the antenna is hitting trees ect go with a steel whip. the ,,springer,,(try google images) is pretty much quarry truck and 4x4 or farm vehicles antenna of choice in england.
if its open highway they still work well as does a decent ,,halfbreed,, type steel whip but you can now go with pretty much anything
so a typical flexi stik type over 3 foot should do ok
as for the valor halfbreed copies
these do vary a lot so go with a known brand (solorcom for example and get the long one.
you may well bend it sometime but just bend it back! also single antennas are just fine! less hassle less expence less repairs and easier to install infact i see a lot of trucks up close and sometimes 1 isnt even connected or its for just the amfm radio. twins also really need to be 9 feet apart so all the twins you see on pick up trucks ect behind the cab is not a good idea. a single up on the back of the cab roof
of pretty much any variety over 3 foot set up right will do better
as for fibreglass cabs you can fit antennas on a 90 degree right angle mount as high as possible and also directly through the roof with a dv or stud mount.
on some i have simply ran a cable from the earth side of the ant mount down to the frame.
on others i have used aluminum tape to cover the underside of the mount area as large as easily possible as if it were metal.
a dv27 on a biscuit tin was the typical new beginners home base set up back in the day
and it worked ok local and dragged in skip. so you dont absolutely need a big metal area but it does help.
i have also mounted one on the mirror and mounted a short antenna straight down on the earth side to make a minature dipole on a fibreglass door.
(you may find a base boomerang antenna works for this)
but you may find that if you look up around the back of the cab you can mount it on some metal there like an air cleaner or stack mount frame a lot of trucks have this
basicly just make sure your getting a good earth of some kind and the ant is up an in the open
.
hope this gives you some options