yes it will be enough unless your bed is all rusted or corroded.
It will be more directional though then it would if it was mounted on the roof.
the direction with the most metal underneath it will be the favored direction of your signal.
so, if you mount it to the drivers side rear corner, you will get out a bit better toward the passenger side front corner.
now, this doesn't mean that you will only reach 1 mile out the back corner of the truck and 50 miles in the favored direction, but it does mean that if you would like to try to help your signal in one particular direction, you can point your truck a certain way in order to do that.
the best things you can do for your mobile installs is to put a common mode choke right at the feedpoint of the antenna, and to bond the different sections of your vehicle together with 1 inch wide copper braid strap. (dont bother using wire, it won't do the job well) bond the hood to the firewall, bond both the cab and bed to the frame, bond the doors to the cab, and bond the exhaust pipe to the frame right near the end of the pipe.
here is a link to some snap on chokes that make the common mode choke easy:
https://www.ebay.com/itm/Palomar-En...hash=item2ceb82cd39:m:m-wJ51yopcNoX6gN5D7Vzfg
the copper braid can be found in many places.
hope this helps.
LC