How to lower a ramp

My dad gave me a wooden ramp he had from his house's previous owner, and it's perfect except for being about 1cm too high. It's made of wood which has been treated with some sort of outdoor protection, and the bit that needs adapting is a like...5x5x75cm wide block under the flat bit of the ramp.

Does anyone have any suggestions for any relatively cheap, easy ways to get it shortened/lowered and then the wood treated so it doesn't rot outside? Bearing in mind that Cam and I are not the sort of people who are any good at this sort of thing, and do not own much in the way of tools. Anyone local who is good at this stuff want to deal with it for me? (worth a shot :D)