Are all Fullsize LWB Camper Shells the same or do they need to be brand specific to fit our trucks?

I've never had a camper shell, but I'd like to get one. I found one that a guy had on a 2nd gen but he wants $460, and I found another for $150 that came off a 96 Ford F-250. Both are Leer brand shells. Just curious if they need to be made for our trucks, or if any FS LWB shell will work?