Service bed dimensions

[Seanext]

I have a K20 LWB of coarse, that I am working on and am thinking of adding a service bed just to be a little different. I can find plenty of used beds but trying to figure out the right dimensions to look for is a little puzzling. I know that most beds are universal mounting to the frame and I will have to build mounts for that. I am looking for overall width, cab to wheel center line, and wheel opening dimensions. And other dimensions that I might need to make sure I get the right size bed. Anybody have one to measure or know the dimensions? I have done searches and can find information for newer truck but not our years.