I currently have an '83-84 block with big cracks in that area that someone tried to repair(Got it for a parts/core along with other stuff).
My friend has a '84 running 6.9 that cracked in that area and was repaired with JB weld. Surprisingly, it worked. Engine got pulled because it was only running on 7, but it still runs(Coolant was not leaking or mixing. Just something wasn't right).
So, yes, a block heater can cause a crack in that area on 83-84 blocks. Pretty sure that does *not* apply to more modern IDIs, and even with a 'weak' block, plugging it in warm should be fine.