I had a '79 Blazer that I used both the brace and the weld-in plates from...
http://www.offroaddesign.com/catalog/steeringkit.htm
Installed both for maximum protection. Easy to install.
There is a proper way to fix the cracks, by the way. Grind the areas to clean them up. Find the end of the crack and drill a hole through the frame at the end of the crack. This will "stop" the crack from traveling. Some suggest to grind along the crack to create a crevice for the weld to penetrate into. Then you can weld the crack.