The first thing I do with ANY crack is to drill a hole at the end or ends of it to keep it from spreading. What size hole will depend on what you are working with. If you can't drill a hole, grind or make a radius at the end of the crack with a rotary file. That's the way the window crack repair guys do it, by drilling a hole to keep the glass crack from spreading, then seal up the hole. How you seal the holes is the same as how big of a hole you need to drill. Depends on the material and the damage. However, the pros often use "stitching" and it works very well, IF they are not starting their learning curve on your part. (This means you most likely need to find an old person.) http://www.locknstitch.com/metal_stitching.htm
Just about anything can be fixed by a person who knows what he/she is doing. Fixing cracks is not an art form, but it does take knowledge and experience.
When I worked at Cupples Products, our 3600 ton extrusion press developed a crack in its cylinder. An outside company was called in to repair the crack and it was repaired as explained above, Drilled holes at ends of crack, stitched and welded. The cylinder was refinished with something that looked a modified floor sander. When done, it worked like new. Had roughly a 40 "bore.
If there was ever a block that should have been made with blind head bolt holes, it's the 400. If Duntov would have had his way, the '63 Corvette would have been a Rochester Fuel Injected 400SB. He couldn't sell it to the bean counters. Could have had an OEM 500+HP SBC Corvette in '63. Think how that would have changed things for the Ford and Chrysler camps.
Like Mr. B. Knuckles says, if the crack breaks into a thread hole, that hole supplies the radius to keep the crack from spreading. No problem. Put an extra drop of gunch around the crack and on the threads, and put it back together.