[Grant Bowker]

The frame rails fo rthe F15 have different part numbers from both the F15A and FGT. Once again I can't tell you what the diffeence is, only that the frame rails for the F15 were unique to the F15.
Your mate is a braver man than I am to take on those cracks. For my welding skill (or lack thereof), those cracks are closer to other things (cross members, spring hangers etc.) attached to the frame than I know how to reinforce. I've also seen quite a few frames that ended up cracking right beside the repair weld. I don't know if that was a result of poor preparation, poor technique, weld heat and metalurgy making the area prone to crack again or something else. The Ford CMP maintenance manual does give advice on frame repair.
If you add reinforcement inside the frame, you will need to allow for the thickness at each crossmember to avoid having the frame wider in some areas than others. you might also run into issues with parts that mount to the frame spannig its width (due to the new width of the frame being slightly greater due to you reinforcement being sandwiched between crossmembers and frame rails) such as springs/axles, cab, bumperreak hitch spring (although it is a loose enough fit that i don't think it would be a problem) rear body (not likely to be a problem for GS bodies given the mounting system). Maybe I worry too much???