The closer the springs get to the center of the vehicle the less overall stability the vehicle will have. I have done stock location, under the frame and just outboard of the frame. The amount of flex inbetween the stock location and moving it in 2" was a decent amount, the difference between that and under the frame wasn't much. I am running a fairly soft spring so under the frame I had too much body roll. moving it 2" helped a ton on the body roll but didn't hurt flex too much.
I would think that inside the frame would give to much body roll and side hill instability.