The replacement battery was slightly longer than the original. I was able to check the fit by standing the battery up perpendicular to the case; the long edge just barely fit. I had to trim some tape off the end and work the cardboard corner a bit. When laying the battery down into the case, I had to take care not to push on and break the plastic board and component to the left of the battery (according to photo orientation - opposite end from the hold switch).
The iPod opening tool seemed too large for the gap between the case and the connector latch since the thin edge of the tool had broken off during case opening. I used the flat tip of the screwdriver tool on my pocket knife. Be careful if you tilt the case during reassembly. The small brackets/bumpers (blue in the photo) can fall out. If they do, just reorient them according to the photo.