According to the parts books (not confirmed by actual observation) each of FGT and F15A had several different frame assemblies with corresponding frame rails. The differences in these seem to have been in mounting holes for different components. Ford or Delco shock absorbers, different steering gears, different winch fairleads/rollers, early and late cab.
Although the changes in F15A and FGT frames seem to parallel each other (except winches are FGT only in the 101.25" WB frames) there are separate part numbers for the FGT and F15A frame assemblies and rails. The difference in the numbers is the prefix designating the vehicle model C011WQF for F15A and C011QF for FGT.
In general, the prefix seems to reflect the first vehicle to use a given part and any other vehicles that use the same part reference the part using the original prefix. For example there are many parts in both the F15A and FGT lists that are prefixed C01QF (either F30 or F60S).
Based on the above (again - not confirmed by actual observation) my opinion is that there is some difference between the frame rails used on F15As and FGTs.
Now for speculation - you may be able to use FGT rails on a F15A without much change (possibly adjustments for mounting a full cab as opposed to FGT body) and F15A rails on a FGT (probably with opposite adjustments for body/cab and added holes needed for FGT use to mount the winch and other equipment).
It might be effective to find/borrow one of each frame, stack them on top of each other, photograph this and hand it to several of your knowledgable friends and ask them to play spot-the-differences. Even better to do this with both side views, top, bottom, front and back as well.
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