Have you had a look at departedflights.com although they don't list 1972/73 they do list 1971 as it shows the following.
LHR depart 12.00
JFK arrive 14.50
JFK depart 16.15
LAX arrive 18.55
Flights operated by BOAC Super VC10.
I should imagine that B707 may well have been used on LHR to LAX by 1973 but a few years before the arrangement with Air New Zealand and the use of their DC10-30's on the route which lasted until 1979.