How many grams of NaOH flakes should I use to obtain 40% NaOH in a 2000ml solution? Flakes have 96% purity. Can I use the density of NaOH to get the grams needed? Thanks for the help!

You want a 40% (weight/volume) solution, so you will need to dissolve 800 grams of NaOH (833 g if you want to correct for the purity) in 2000 ml of solution. I can't see how the "density of NaOH" will help you; weigh it out like any other solute. The density of the 40% solution will help you determine the volume of water you will use.
All this is easier said than done unless you have a funnel with perforated sides designed for just this purpose. The dissolution process is VERY exothermic and the rate of dissolution slows dramatically as the concentration increaases. You will need a good magnetic stirrer and plenty of patience. Make up a more concentrated solution than 40% so you can add the water to get the final volume. I wouldn't worry excessively or obsessively about the final concentration -- after all, a 40% solution isn't going to be used in titrations or for other precise techniques.

Thank you!! XD