For me, the 27" tire that characterizes the '70s was the Michelin Fifty. 27x1 1/4. Gumwall, fine chevron tread, consistent quality, 70 psi.. In the later '70s you started to see 27x1 1/8 tires and Avocet appeared with their slicks. My go-to for '70s bikes with 27" wheels is the beige sidewall Panaracer Pasela though we also have two bikes with beige sidewall Schwalbes (not sure which model, but they were cheap, like $19 on sale). The Schwalbes have a heavier, more robust look to them on both tread and sidewall. We use those on the bikes that are off-pavement more.
For a more slick tread, there are the Continentals but most what is offered now all seems to have a dark or black sidewall. I also get the impression that they are trying to cut back to one set of molds and make a single 27" tire that is in between 1 1/8 and 1 1/4. Some people grouse about Continentals but I've used a lot of them and don't really have any complaints.