15
That the car now cranks and runs correctly tells us that the battery and the cables are conducting electricity acceptably. I see no need to cut and refasten either cable, especially (as you note) the (+) cable may not be long enough to allow connection after it's made shorter.
The minor corrosion visible in the photo is external to the clamp surfaces, ...
10
From the images I'm seeing very little corrosion. I think your bigger issue is the clamps themselves seem to be worn out. While you say it is "cheap and easy", you also ask "where do I cut them at" ... the answer to that is, you don't. There is the nut on top which is there. This, when removed, separates the battery cable from the clamp. ...
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