If I were in NY I would post a picture of a friend's 1981 4x4 that broke on the driver's side through the oval cut in fords_forlife's picture.

 

see my first post.
yes only 80 and 81.

 

I believe they start right at the back of the cab and go to just above the rear axle but I haven't looked in a while

FORDS_FORLIFE

 

Must not be all 81's for my frame is solid. My family has owned it since new. It is a 1981 F150 4X2, 302, AOD, Flairside.

Wow, those are pretty big holes!
Kenny

 

I've been told the strength of the frame comes from the top and bottom channels.
Ever notice most items are atached the the sides of the frame.
Drilling (or rot) in the upper or lower channel will destroy the integrety of the frame.

 

Yes the integrity comes primarily from the upper and lower framerails however if there is nothing between them it seriously compromises the frames integrity trust me i have an 81 f250 that twists like a pretzel if run over a 2x4 so bad even in some cases that you can't even open the doors it is also starting to bend at the oval hole behind the cab bought new never hauled towed or offroaded just junk plain and simple