I think the bigger "issue" may be the center seat belt anchor point for the shoulder portion. In a collision there is going to be a lot of force there, and the center of the cab roof wasn't designed for such loads.
If the reel is anchored to the seat itself, as long as you have more than plain floorpan for seat attachment, you're probably good. Keep in mind that these trucks (like most ford trucks until the mid-late 90's) have smaller stock seat bolts and somewhat weaker attachments since the belt was a separate floor connection belts aren't anchored through the seat base like on newer seats. Hence why when I put 2000's seats in my 1995 Ranger, I kept the old belts and removed the seat-mounted receptacles (which weren't compatible with my stock belts anyway). If possible, you'll probably want to do the same for the outer seats (or be prepared to do some serious bracing to the seat mount points).
1984 F150: 300 L6, AOD, RWD. EEC IV / TFI, Feedback Carter YFA Carb. Stock everything but radio (for now).