How would you make a 400 mL solution of 0.7 mol/L NaCl from a stock concentrate of 2.0 mol/L

I don't call this stoichiometry. Use the dilution formula of

mL1 x M1 - mL2 x M2
400 mL x 0.7 M = mL2 x 2.0 M
Solve for mL2. Add the mL2 volume to a 400 mL volumetric flask, add distilled water to the mark, stopper, mix thoroughly, label.