Well. I found the tripartite BA/QF/SQ timetable for Winter 73/4 and needless to say, there is no straightforward answer.
There are two Monday arrivals in Perth. One is BA724 which left LHR at 16:45 on Sunday and transited Frankfurt, Bahrain and SIN before arriving PER at 22:55
The other was QF740 which left LHR at 17:30 on Saturday and transited FRA and BAH on the way to SIN. This connected with QF726 which arrived PER at 02:40 on Monday. This involved switching between a 747 and a 707.
The only Vienna call in the pattern would not have fitted – it was on a Qantas Monday departure from LHR. Tehran would not have fitted either, although QF stopped there most days these flights did not serve SIN, they used Bangkok or Kuala Lumpur instead.
Which leaves Bombay … as far as the Kangaroo Route was concerned, this was served only by Singapore Airlines, on their Monday and Friday departures from LHR. The Friday flight (dep 16:00) transited AMS, FRA, BAH and BOM arriving SIN late Saturday evening – but still early enough to catch the QF726 to arrive in PER on a Sunday. So the only way that would fit would be if you made a 26 hour stopover in SIN and caught the following day’s QF726 (as above). This would have been a 707 throughout.
Over to you …
The SSK
P.S. I said that the ABCs were in the cellar of my office, where they are archived for eventual research purposes. My wife is blissfully ignorant of their existence and my office is no doubt blissfully unaware of their value - there's more than 50 years' worth.