As from the first day of January, nineteen hundred and forty, section thirteen of the [10 & 11 Geo. 5. c. 18.] Finance Act, 1920 (which imposes duties of excise in respect of mechanically propelled vehicles) shall have effect as if the paragraphs set out in Parts I and II of the Eighth Schedule to this Act were respectively substituted for paragraphs 1 and 6 of the Second Schedule to that Act.