The label says 34 watts. The datasheet says 18.17. Which do I use?

Use the rate your application runs at, and read both from the datasheet rather than the label. At the 15 minute rate this block gives 18.17 watts per cell. At 5 minutes it gives 36.33. The 34 in the part number matches the 5 minute rate, and CSB's price list agrees, listing 36 watts per cell for this part. Every other HRL part badges on the 15 minute rate, so this one is the exception and it catches people out. If your specification names a 15 minute window, 18.17 is your number.

Can I use two of these in series instead of a 12 volt block?

Yes, and plenty of equipment is built that way. Two in series give 12 volts nominal and the same current, so 18.6 amps at the fifteen minute rate rather than double it. The charger has to be a 12 volt unit holding 13.5 to 13.8 volts across the pair, not a 6 volt charger on each. Match the two blocks by date code and replace them together; a mismatched pair in series ages to the weaker one.

What charger voltage does a single block need?

6.75 to 6.90 volts on float at 25°C. That is the figure worth checking first on a retrofit, because a charger set for a 6 volt flooded battery often floats higher and will dry out a sealed AGM over a year or two. If the equipment was designed around a sealed block, it is almost certainly right already. If you are converting something, confirm the charger before the battery goes in.

How is this different from a standard high rate 6 volt block?

Plate design and expected life. The Long Life construction is rated to eight years of standby service at 25°C and 10 to 12 on the Eurobat basis, against the five years a standard high rate part carries, and CSB backs it with two years rather than one. In exchange it costs more to buy. On emergency lighting that gets inspected rather than replaced, the longer life usually wins on total cost over a decade.