I was there...well at BHX anyway.
I can't recall runway limitations being a factor at BHX, though taxiways certainly were as there was no parallel taxiway then, and with landing on RW33 it was a full backtrack, rather than the normal right turn at the western taxiway.
Unless I'm mistaken, nothing much changed with BHX runway until the starter strips were put in when the new terminal was built in the mid eighties, and between 1978 and 1984 I'm sure that 747s operated through BHX, though possibly only diversions. Even then some must have been refuled before heading off to LHR.