If someone used an impact to change our a starter on a mid-90's Z-71, could that alone crack the block? Can the block be welded?
If a crack developed in the starter mounting boss area, using an impact alone would not have caused an issue. All blocks before 1998 were still Gen I small block Chevys (SBC) engines. These were cast iron blocks, so very tough and resilient. If an impact would have caused any issues, it would have stripped out the threads before it would have caused a problem with cracking the block.
If it is the starter mounting boss which is cracked, it is more than likely caused from not installing the starter correctly. That would most likely be not tightening the bolt all the way or cross threading it, which wouldn't leave the starter sitting flush against the block. This would cause unusual stresses on the block when the starter was used.
Can it be welded? Yes, but it's not something just anyone can do. The block would have to be removed from the vehicle in order to weld it correctly, because the crack has to be drilled where the crack starts (to stop the crack progression), the the crack itself has to be V'd out (using a bit to create a V where the crack is at), then the block heated, and then welded. It isn't something a qualified person would want to do with the engine in the vehicle. Depending on the crack, brazing it might be a better option, but that again would be left up to a qualified person to determine. Brazing would be best done with the engine out of the vehicle as well.