Why size against 414 watts per cell rather than the 390 in the name?

Because 414 is what CSB measured and 390 is how the model is listed in the catalog. The published figures are 414 watts per cell at the 15 minute rate and 700 at the 5 minute rate, both to 1.67 volts per cell at 25°C. Across six cells the 15 minute number comes out near 2,484 watts for the whole battery. A worksheet built on the model number will under-size the string by roughly six percent.

Will an HRL12390W fit where an HRL12330W was?

Check the length first. The 390 is 13.50 inches long against the 330's 12.15, so it needs 1.35 inches more shelf depth. Width and height are within 0.13 inches of each other, and the terminal hardware is identical, so cable lugs and torque carry across unchanged. If the shelf is tight, send its internal dimension and we will confirm before anything ships.

What does the lower internal resistance actually get me?

Voltage stability under load. At 3.00 milliohms the 390 sags less than any other block in the range when an inverter takes a hard step, which means the string holds above its cutoff longer for the same draw. On a long string the effect compounds, because every block's resistance is in series with the rest.

When should I step up to the HRL12540W instead?

When the 15 minute column still comes up short, or when shelf space is the limit rather than weight. The 12540W delivers 541 watts per cell at 15 minutes and 135 Ah, in the same 13.50 inch length and 6.69 inch width but standing 10.85 inches tall instead of 8.54, at 96.76 lbs. So it buys capacity without more floor area, and it costs height and 24 lbs per block.