Spencer would have also done very well at the 70.3 distance. He had some success at the IM distance, but I agree with you that the 70.3 distance would have been his best chance at winning events. He was a fantastic cyclist, had good speed in the water and was a very strong runner!
Simon Lessing also comes to mind when thinking about triathletes that attempted the IM distance with some success. He would also have been better at the 70.3 distance.
Both had the physiology to excel at 70.3 but Spencer would have had to have gone down that route in 1995, and in recent interviews he mentioned that after his father died in 98 he lost some motivation. Assuming he brought the approach that got his two world champs in the non-drafting era, then he would definitely have been a contender.
Lessing was focused on Sydney, I don't think he cared much for racing long but had to for the dough.
They both would have had lots of wins at 70.3 in that particular time frame, but compared to the lightening we saw from the top three mutants in South Africa I suspect less so if they were competing now.