no, mk3.5 is not the same as mk3 hence why its called a 3.5 :roll:
certain things on the mk3.5 were different to mk3 parts just before the cross over to mk4 happened.
runners were one of them, same floorpan layout as the mk4/puma but could still fit older fiesta seats to them
mk3 runners dont fit to the puma floor pan
to be safe find an n reg fiesta as that will deffo be a 3.5