I've been working on my tailgate as well - but I'm not sure I have the same gap as you describe along the bottom edge. On my 65, the inner and outer tailgate sheetmetal join (or are squeezed) together a few (?) cm from the bottom edge. Along the bottom edge, there is a rubber strip (like a wide rubber wiper blade that runs the length of the bottom) that is held in place by a strip of metal along the bottom edge of the tailgate. The tailgate at this bottom edge is pretty rust free, so it does not look like any water was trapped at the bottom (although I could see how this could happen easily), nor does it look like Volvo designed any drain holes.

There is a narrrow tray beneath the tailgate assembly that is designed to catch the water that gets behind the tailgate, collect it in the grooves and drain it to the ground. This tray had some rust that needed my attention.

I must be missing something here (how many different ways can you describe the bottom of a tailgate anyways?) - maybe we can trade tailgate pictures.

waggin.