[VAC] Width of structural bows?

[Rob Super <rms@xxxxxxxxxx>]

Can anyone tell me the exact width of the upper flange of the structural 
bows (the aluminum ribs that run up the sides and over the top) inside 
the roof of our '73 Safari?

I believe these are U-channels oriented with the base of the U facing 
the front (or back?) of the trailer, and I'm guessing that the flanges 
of the U are on the order of 1-1/2 to 2 inches each--but I want to know 
for sure before drilling. I'm installing solar panels, and the rivet 
lines on the roof are nominally 48" on center, as is the distance 
between the mounting holes on the solar panels--but each measurement is 
a bit off (in opposite directions) so I need to make a bit  of an 
adjustment and want  to be sure to hit  structure.

TIA,

Rob