In 1969, the Legion was still appearing in Adventure Comics (where they'd been appearing continuously throughout the 1960s), but by 1970 they had been moved to Action Comics as a backup feature. They were later moved to Superboy as a backup (with #172, I believe), and then Superboy became "Superboy and the Legion of Super Heroes" with number 197, in 1973.
There was also a 3 issue, reprints only LSH title in 1973.
No, I'm not consulting any reference material, I actually am that big a geek.