Can you safely install high-pressure sodium wall packs indoors?
Other than quickly reducing your visitors (wow, remember when we had visitors?) to those like-minded individuals who love HPS light, there's no issue with putting a wet-rated &quot;exterior&quot; fixture indoors, unless the instructions forbid that. Indoors is generally a less extreme environment than outdoors, so a fixture that operates safely outdoors will operate safely indoors.
Other potential downsides: I think 35W is about as low as you can go with HPS bulbs that I've seen, and there IS the fact that they take 5 minutes or more to turn all the way on.
If the wall pack is designed to mount to a junction box, you must use one. If the wall-pack design incorporates a junction box you don't need another. You don't need a GFCI on lighting circuits if they are hard-wired unless they are too close to a tub or shower - you do if they are cord-and-plug, or too close to/over a shower or tub.

For a similar (not the same, though it is the same color temperature) color and faster turn on with lower wattages, there are 2000K LEDs available.