Engine block dont just crack, somthing has to make them crack, like freezing over without proper glycol mixture. Your blwing a headgasket may have caused the crack due to the engine "hydraulic" when water gets into the cylinder it cannot be compressed, somthing has got to give. The 3800 series block is very strong due to it's "y" design, and cross bolted main bearing caps, (best type of four bolt mains).
If your going to be doing any type of hard driving, first on the list of modifications that I think should be done is opening up the top piston ring gap at least .005-.010 for n/a applications, and .015 for supercharged and turbocharged engines, but this number really depends on the type of pistons you are running, and how much heat (power) you plan on making wiht the engine.
Like everyone has stated, fast=money to be spent, and you have to EXCPECT things to break when you drive them hard because eventually somting will let go, no mechanical device is perfect.
