Just think about that Blazer cranks but wont start
 

Just reinstalled 4.3 L Vortec out of my 1998 Chevy Blazer LS said I rebuilt it turns over but it won't start haven't got the exhaust on it yet just open headers so my O2 sensors aren't installed in my temp sensors not installed what could keep it from starting

Fuel Psi right, spark at the right time ? And if it's the temp sensor for the computer 90% of the time it won't start with it unhooked,