Planing will work, but the end grain work requires a good plane iron, extremely sharp and perfectly adjusted in the plane to make it anything other than a wretched chore. If you aren't experienced it may not be a good route to take.
Since this task requires a multi-hobby/interest approach you might check if you or a friend has a mill. Something that goes through aluminum/steel laughs at end grain. You could have a surface ready for finish sanding in an hour or less including setup. If you go out to a shop to do this your biggest issue will be being sure they don't contaminate your block with coolant, machine oil and other nasties.