jorb & drdoug, you blokes have made my day! I've been trying to find an old mechanic who may have worked on these vehicles, but no go at present. I will crawl under the truck tomorrow & see what I can find in the way of slots, dowel pins, etc.
I am a retired civil engineer by trade, and also managed a large earthmoving fleet so I spent a good deal of time in the workshop watching, listening & learning. An old boss once told me that anyone could do any job, as long as they were shown the right way. You people have done just that, so I feel confident of tackling the job now.
As I have previously posted, I have purchased a step side pickup bed from Jim Carter, the panels are at the paint shop at the moment, and I am hoping to have it all together by Easter. I'll stick a picture on the forum when it's done.
Once again, many thanks for all your help & advice.