Every covenant and power to be implied in any instrument by virtue of this Act may be negatived or modified by express declaration in the instrument or endorsed on it; and in the plaint in any action for a breach of any such covenant it shall be lawful to allege that the party against whom or against whose legal representatives the action is brought did so covenant, precisely in the same manner as if the covenant had been expressed in words at length in the instrument, any law or practice to the contrary notwithstanding; and every such implied covenant shall have the same force and effect as if it had been set out at length in the instrument; and where in any instrument there shall be more covenantors than one, such covenants as are by this Act declared to be implied in instruments of the like nature shall be construed to be several, and not to bind the parties jointly.