6-speed cranks, 8-speed chain/cassette compatibility?

I've got a few sets of old but useable freehub wheels with newish 8-speed cassettes. I tried to use one with an old early 70's 5-speed cottered crank Peugeot. Can't remember the width of the old chain I tried (8/9/10 speed?), but it was narrow and got stuck between chainrings. So, that was a minor...