I think it's pretty common to have to drill the frame if you just use the multi purpose 'L' brackets. It costs a few bucks more but the vehicle specific brackets are pretty nice as they use factory holes. They also tie the hole thing together better IMO instead of each of the 4 L brackets being seperate.
Back to the OP, can you tell if it had rails across or lengthwise with the bed? maybe by indentations in the bed corrugations? I know there are some rails that run lengthwise. I believe DSP and older Husky models used those kind of rails.