As a matter of fact, I'm doing that to my '67 Ford F-100 right now. Take a look at http://www.oldsmobility.com/F100/my67-p18.htm
for some pictures.
On my pickup, the drip rail is basically an 'L'. The short leg is the drip rail, the long leg is pinched and spot-welded between the upper roof panel and the lower cab portion. The area just above the drip rail on my roof was rusted out, so I'm having to replace it, using a donor cab, but cutting a portion of the roof panel off to give me access to the spot welds.