On the Mercruiser 3.0, many motors/blocks are replaced, in my opinion, unnecessarily. They almost never crack to the inside of the block when frozen, if they crack at all. Many other times they simply pop the freeze plugs, sometimes giving the appearance of a cracked block. The solution in this first case, would be to use JB Weld for cracks, carefully applying a nice seam above the crack, and letting gravity carry it to the appropriate place, then doing it a second and, perhaps, a third time. Outside jacket operating temperatures never exceed JB Weld's limitations. Fixed. The solution in the second case, obviously, is to replace the freeze plugs. Google them, purchase, get a perfectly sized socket and extension, and gently pound the new ones into place.
All of that said, I have heard that the 4.3s can crack to the inside and/or outside. Check the crankcase oil for milkiness, and indication of water, in which case you are schtupped. If not, proceed as with a 3.0 . . .
