Some bikes with 126mm OLD at the rear triangle cannot readily accommodate 7 speed FW's.
I found that out when I tried to install a 7 speed freewheel on my 1984 Peugeot PY19FC. The smallest cog could not clear the drive side seat stay bottom end.
It turns out that the frameset was designed by Peugeot to have seatstays are a bit closer together, above the dropouts, compared to other bikes with 126mm OLD spacing. So I'm stuck with just a 6 speed FW on the bike.