It looks like frame rails are in my cars future...

I don't own a convertible but I do know that the rails are very different. In convertibles, the frame rails have much more weight on them to support the car than a hard top does. I was looking at a '67 Corvair Monza convertible once and this was also an issue. So yes, the metal does have to be a thicker gauge for a convertible. It is very crucial to the structural integrity of your car so don't put the wrong ones in.