I have had my buddy weld a few up for me...nothing to it he says(coming from a PRO). So with that said make sure you take it to someone thats trusted welding cast pieces and you will be just fine.

How bad was the crack that he repaired?

From right of the starter to about 8 inches behind the top right bolt was the nastiest and its still holding. Just make sure a pro does it with a really good tig welder.

If it is done right...weld and go.

Even seen 500+ hp cars with welded trans, all depends how bad

I would keep searching for an uncracked transmission unless this thing is crazy cheap and you can't pass it up. Or if ya can't find anything else. And even if I bought it to get me by I'd still keep searching for an uncracked bellhousing. Sometimes finding parts is impossible though and ya gotta do what ya gotta do.