Would the 727 Adv have had the range? Have they ever been timetabled for non stop transcon routes? I can't remember any from my misspent youth looking at ABC timetables.
The 738 is certainly around 40 knots slower than 747, Tristar & DC10s that used to fly the routes but those days are long gone (sadly). Times are now slower owing to smaller aircraft, traffic congestion & schedule padding. SH European flights to/from Heathrow are timetabled to take longer now than 737s took 20 years ago (you would expect them to be slower than "hotrod" Tridents and CV990s).