Is it possible to clean up / deglaze the cylinder bores with the heads off and the lower end still installed? Looking at doing a top-end rebuild on a tired 318 and don't want to take the engine out if I can avoid it.

It's done all the time; just be sure to wrap the rod journals to keep swarf out.

Remove SOME of the ridge with CAREFULL use of a ridge reamer (if needed). Pull dont push the cutter around the bore and changing your body position often so you are evenly pulling the cutter around the bore. Keep the tool flush with the deck (a helper is nice). It does not cut the ridge ID perfectly so do not go to flush at any point as you will likely be past the ridge/into the wall at another point. Stop & check often. Prep the bores with a brush research mfgr dingleberry flex hone with a 320 grit. get some plasma moly file fit rings. As Kunkel said protect the journals with some masking tape. Get each journal at BDC. You can soap and water scrub the bores with a minimal amount of liquid & keep it out of unwanted areas with some care & soap and water is the best way to get microscopic grit out of bored/honed cyl walls which will destroy a new ring job. Read "breakin secrets" at www.mototuneusa.com

I learned a new word today.
Had to Google it.

NO, and also the use of a ridge reamer will sometimes kill a block if not used right.

MLR426

Instead of soap and water use transmission fluid and paper towels. When done with honing, pour some trans fluid on a folded paper towel and wash the bores. Do this to each bore. After washing with the trans fluid, continue to wipe each bore with a clean paper towel until you stop getting residue on the towels. It is how we did it before engine assembly in the machine shop I worked in. It may take an hour or more to do it right, but you can be sure the bores are clean and there is no water getting in the engine.