Y'know, for most people, the Quarter Horse is pretty much the poster child for the term "easy keeper," so I have to wonder - what was the problem with those two you had, that they couldn't maintain weight on 24/7 pasture? Age, teeth, parasites? I mean, I have had to pull horses off of pasture for at least part of a day because they were getting too fat, and we don't have particularly good grass around here. Whatever; it's water under the bridge at this point.
But whatever his true ancestors are, one thing about a mustang - he is by nature a survivor. Mother Nature is heartless; you get it right, or you're dead. This is (as near as possible) a horse as a horse was meant to be; the worst thing you can do to a mustang is to lock him up and coddle him.