It's INCH pounds which is usually a 1/4" torque wrench as the 3/8" wrenches are in FtLbs. Forty InLbs isn't a whole lot and is usually snug + 1/4 turn (kinda like an oil filter). That shouldn't bend the board unless it's a 3 layer board or less. Most water blocks I'm familiar with just have springs and the brass nuts just hold tension on them, which the Apogee I used had as well. As long as you don't fully compress the springs then the specs should be inline.
The metal backing plate is always a good idea too. With the Lexan pieces, they tend to bend as the piece heats up. Here's a DD Lexan s775 backer that's bent and allowed the 6-layer board to bend too.
The backer should be the main support and keep the board from bending. If the backer bends, it's not doing it's job.
I lost the pic, but have a pic of a Foxconn ELU board bending just from a coolermaster V8 aircooler even with a metal backing on too.