Weren't the 747-SP also beefed up and structurally reinforced ?
Just the opposite. They were lightened versions of the 747-100. Instead of triple slotted ailerons, they had lightened double slotted ones. The fuselage was shortened by 50 feet, and the tail was increased in size. The landing gear was also lightened. The PW bought two 747SP's for their engine test bed program -- one to fly, the other for unique SP parts and other spares. I think the SOFIA plane also has spares taken from a second former United SP. I know they took the entire empanage from another SP and used it for a mockup of the SOFIA plane so the process of cutting a hole and mounting the infrared telescope could be debugged prior to cutting on the working plane. Supposedly the mockup is now used for training.