Towing 5th wheel with short bed truck.

A sliding hitch is an option. Is the hitch that came with FW a slider. By extender, do you mean a replacement extended pin box? Does the FW you bought have rounded corners on the front cap? A sidewinder pin box is an option, if you want to use non sliding hitch.

FWIW, I've towed for many years with standard hitch 6 1/2 foot bed, with no issues. Being aware, when turning short, or backing, is good, just like backing a TT, it will hit at some point.

Jerry

you need a slider hitch that you unlock and it slides the pin towards the back of the truck to give you clearance when in tight situations and slides back over the center of the rear axle for travel so you don't hit your cab or break your trucks back window . Depending on the hitch you received with your trailer there may be a slider base available for it and you can use the same hitch head . you can be as careful as you can but sooner or later you will damage your fifth wheels front cap or your truck or both . good luck
Mike

Adding an extended pin box to a trailer not designed for it could end in disaster.