Why is the 5700 lower resistance than the bigger 6700?

Because internal resistance follows plate geometry and cell construction rather than capacity. CSB publishes 2.22 milliohms for the 5700 and 2.33 for the 6700, and both figures are theirs. Do not assume the larger block wins on every specification: if your application is resistance-sensitive on a long string, the 5700 is the stronger choice even though it is the smaller unit.

What does low internal resistance get me?

Voltage stability. Every block's resistance is in series with the rest of the string, so on a 40 block bank the difference between 2.22 and 2.33 milliohms is about 4.4 milliohms across the string. Under a hard step that shows up as less sag, which means the bank holds above its cutoff voltage longer and delivers more of its rated runtime.

Will it fit a shelf cut for a group 31?

The case is standard group 31 at 13.50 by 6.69 inches and stands 8.53 inches tall. Check the height against your shelf spacing including the terminal hardware and cable lug, which add above that 8.53. If you are replacing a group 31T unit the footprint matches but the height will be lower, so the shelf will have room to spare.

How do I set the commissioning baseline?

Take an impedance reading on every block when the string goes in and record it against the block's serial. Treat CSB's 2.22 milliohms as a nominal for a new unit rather than a pass mark. What a maintenance program watches is drift away from the reading you took on day one, so the number you record at commissioning is worth more than the datasheet figure.