Wrong bed

[Phillip Pennartz]

I have a 77 C30 cab and chasis truck. The truck had a service bed on it and the PO put a long bed on it. But in order to get the tires to line up with the wheel openings the bed and cab must have a three inch gap between them.
I'm guessing that I'm dealing with a longer wheelbase than a regular truck.
Is there a special bed that I need? I ran the VIN it says nothing about wheelbase. (CCT337S178428) And there is not a sticker in the glovebox with the RPOs. Also the rear bumper will not fit with the bed in way.
Also truck has dual tanks so would like dual filler doors. So am I stuck with a flatbed or is there a truck that I need to look for to get a bed from?