I'm going to guess, from the nature of the question, that the OP is a new brewer.
IMO, new brewers should stick with established recipes at the outset, see what they produce, and only then, begin to explore what might be changed to alter the outcome. For instance, increasing the grist will increase the gravity, but if you want to change the body, then you should be looking at mash temps.
No offense intended, but it sounds like taking a stab at something, anything, in the hopes the outcome might be better. It might be more valuable to find established recipes in the style desired and then work from there.
One more question: OP is trying to produce "fuller body." What, in a dark ale (what's a dark ale?) is going to typify "fuller body?" What, specifically, are you trying to produce? Better mouthfeel? Maltier? More flavorful and satisfying finish?