I agree with V-grooving it only if it's bigger than 1/8" and I would definitely drill both ends to distribute the load that caused the crack in the first place. Pre-heating like they're talking about is only necessary if using a 110v mig welder, 220mig or tig shouldn't need pre-heating.
I have to agree also that it wasn't the coil-overs that caused this. If it was the coil-overs it would either be up near or on the shock mount or it would be near the opposite leg of the a-arm where the coil-over is mounted, not on this end.
Given that it's the lower side of the frame and it's location, I'd say pothole or some other hard drop on that tire.
