If you notice in the front a rear photos, the top of the shoulder on the GW torso is roughly level with the outer surface of the armour (not the flange) on the Perry arm. So at the very least, they need to be moved up a bit. That's where the putty or infill I mentioned before comes into play. The arms are recessed under the armour, so this was the best I could do for a temporary assembly.
The other issue I have with scale is considering the arms inside of the armour are going to be much smaller than the legs. So it will either look like they have shriveled up arms, or extremely meaty thighs/calves/ankles (the puff & slash parts help hide this, but not the rest.)
I'm not trying to tell anyone else what to like. Just explaining my own issues with mixing them.