I've got a crack in the same place you have described, simply due to the fact that its had a damn good thrashing over the years!
If you drill a hole where the crack ends, to stop it going any further, then just mig the crack up it should be fine.
Seam welding the engine bay is a good move, its something i'll be doing next time the engine comes out.

Wouldnt worry about it too much, thes cars are getting old now so its to be expected