Maybe I'll give this one a go.
I would stop feeding the dog regular meals in a bowl and start hand feeding for good behaviors - the dog is only fed during training sessions (this can include walks) or when they offer you good behavior, like sitting politely for your attention. If the aggression is severe enough that you cannot have the dog eat from your hand then toss the food a little ways away from you. The dog will learn that you mean food, not the removal of it, and that it will get to eat for good behavior - here you can work up to hand feeding.
Resource guarding of food especially is a type of fear aggression - the dog is afraid that you are taking away its food so it postures and says "back off." What you need to do here is change the dog's emotional response. That's what opperant conditioning is all about
As part of your training sessions I would recommend a 'drop' command (anything that means "give me what you have in your mouth, or what is right beside you"). Work with low value objects and do the exchange system (trade object A with object B, but object B comes with food as well) - the dog will learn that when you ask for something, it does not mean they are losing it and that something even better will happen. Gradually work up to an empty bowel, then one with non-food objects in it, then one with very low value food items etc.
Get multiple people in the shelter to do this with you to proof the behavior.
The dog will be conditioned to associate good behavior and the presence of people with being fed and to accept people tampering with their food and their food-bowl.
If you were to use P+ to curb the aggression, you would run the risk of either an escalation of their aggression, or a masking of the behavior. Maybe the dog would stop being food aggressive around you, but what about when they go to their new family? Because they are still afraid of their food being removed (and were only curbing their aggression because they were afraid of your punishment) they may start that behavior all over - but escalate it from the start to try to keep them away.