If you aren't using CO2 a single stingray 2 should give you plenty of light in a 40b. According to
the 20" model in a 10 gallon tank measures 60PAR at the substrate 12" away from the light. Since a 40B is around 18" high you should expect to get around 30PAR (inverse square law for a linear light) which is good enough to grow most low to medium light plants. You can always add a second one in the future if you want additional light or better coverage, since the 40b is a very wide tank and you may get shadowing on the front or back with just 1 light.
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