With regard to 411A's comments I think it depends on whether one is operating to FAA or UKCAA rules. The UKCAA engine out ferry rules were much more penalising than the FAA's. I worked for an operator under CAA rules and we only 2 engine ferried Tristars once or twice in the early days as it was all too hard. Used the Tristar 4th pod initially, then bought a 747 freighter which was much easier and also got RR to position loan engines strategically around the network and never 2 engine ferried again.
For Gas Path, typically an aircraft is only certified to carry the same type and model engine in an engine underwing pod as fitted to the aircraft. Some JT9 models were more or less identical so a JT9 powered 747 could carry several variants of the JT9D.
At least one RR powered 747 operator got certification to carry Tristar RB211-22B's on its RR powered 747 using the 747 5th pod but this required a dedicated test flight to get the FAA to approve it.
DC8 could also carry a 5th pod similar to the 707.