IIRC, the "Swiss cheese" frame was only used on F100's, not on F150's. If his truck currently has a 351M (assuming it is original), then it is not likely to have a Swiss cheese frame.

 

The so called swiss cheese frame was used on all 1980, and early 81, F100/350 trucks. It's under my F150, I've seen it under a couple F250's in person, and there's a couple people who have posted pics of a F350 on this forum with the holey frame. There's even a fairly long restro thread about an 80 Ranger 4x4 that showed the swiss cheese frame.
The frame doesn't suffer from any structural issues, aside from a crash. During a crash, it can fold over itself at one of the holes. I wouldn't want to be using it for plowing duty, but they're plenty strong for most other working conditions.

 

my dad did this with his 84 f250 4x4, he got a 7.3l out of a 90(something) f250 and put it in his, not many things he had to change, the gas tanks did not need to be changed, the gauge cluster did, and as far as weight had no effect.