I really think some JB weld could fix that problem right up.
That stuff is great, I use it on everything and anything, except hair gel. That wouldn't be a smart way to use it
.
That should repair it right up, allow it to sit for 24 hours after applying, sand and it'll be gone in no time. That crack is not substantial enough to have any major problems. From what I can tell, it's only cracked on the bolt hole on the manifold, and NOT the water crossover. If it had cracked anywhere on the main part of the water crossover itself, I think I might be a little concerned.
Also heliarc might be a solution, but I am not all that familiar with it. I had a cracked TH400 case I needed repaired with similar damage from being dropped at one point. The heliarc repaired the crack and it's hard as nails.
Hope I helped, I know the frustration. Try installing a brand new manifold, pulling a valve cover off, and having one of your valve cover bolts slip off...roll down the oil drainback passage, down yout lifter valley, and finally resting on the windage tray. Now that is not fun.
Anyways, no fear. I've made lots of dumb mistakes on my Pontiac and always find some way to get myself back on track.
Take care,
Bryan