Can I swap a 5700 string for 6700s in place?

The floor area is identical at 13.50 by 6.69 inches, so the shelf footprint works. Height is what stops it: the 6700 stands 10.85 inches against the 5700's 8.53, so you need 2.32 inches more clearance per shelf plus room for the terminal hardware. Weight per shelf also rises by 24.24 pounds a block. Send the rack drawing and we will check both before quoting.

Why does the 6700 show higher resistance than the 5700?

Internal resistance tracks cell geometry and plate construction, not capacity, so it does not fall in a straight line as the blocks get bigger. CSB publishes 2.33 milliohms here and 2.22 for the 5700. Use each block's own published figure when you model string voltage, rather than interpolating across the range, and take your own commissioning readings once the bank is in.

Is 140 Ah the number I should be specifying from?

Only as a cross-check. A UPS discharges hard for minutes, and capacity is measured drawing gently for 20 hours, so the two describe different behavior. The 651 watts per cell at 15 minutes governs the ride-through you will get. Use the capacity figure to compare this block against a competitor's datasheet, and the watts figure to size the string.

What does the 30 second rating tell me?

It covers the first instant of a transfer, where motor inrush and power supply charging can push the draw well above the steady load. CSB publishes 1,467 watts per cell at 30 seconds for this block. On loads with a heavy inrush component it can be the figure that decides the size, even when the 15 minute column looks comfortable.