If I delete an existing chunk with MC Edit in my pre-1.8 save, will the new landscape in 1.8 line up with it?
"How abrupt" depends entirely on the particular chunks in question, and is impossible to predict to boot. There's really no way to know until you try for yourself.
If you're unsure you want the potentially very obviously misaligned chunks, you can always backup your world before deleting it, reverting if the border does not line up particularly smoothly.