... and I would add: If the device with x18 /x36 architecture is otherwise suitable, available, and of right price (and you do not need error checking or correction (parity / ECC)), you can use them and leave the extra bits unused. However, you should use for example a weak pull-up or pull-down on those extra data lines to prevent them to settle halfways between logic levels: It is not good - it may lead to excessive power dissipation and reliability problems.
Ted