I'm not so sure that Boeing didn't have structural problems with the 707 or even the 747.
Its a long while ago now, but I have vivid memories of working mid winter through the nights in H96 on the 707 fin mount replacement on what would have been a 4 or 5 year old fleet? Later ferrying 707s to BOAC for spar repairs.
And on the 747 the repairs to airframe behind the FEs panel (section 41?)
Wunwing