Conversion tables are only an estimate and approximate. Over that distance it cannot be ascertained to the degree you suggest, of tenths of a second. We see from other record-holders for both distances - like Elliott, Ryun and El Guerrouj, that the differential could be anywhere between 17-19 secs. The fact remains, Walker's mile record was far better than Bayi's best mile time. His 2k record of 4.51 in which he ran the last 1600 in 3.52 is also far better than any performance (in terms of time alone) than anything Bayi ran.
Nor would I agree that Bayi would have run faster in '74 if he had been drafted. He never ran faster than when he led. I do however think Walker would have beaten him at Christchurch if he (and the field) had been prepared to stay closer to Bayi. I don't think Walker at that stage of his career believed he had a wr run in him - he was a comparative novice over the distance - and like the rest of the field probably thought Bayi's pace was suicidal - for him.