As was stated, SII were Buick Motor Sports parts produced primarilly back in the 80's for Buick sponsored racing applications.
Grand National Cup Stock Cars and many other stock car classes.
Indy Cars
Indy Lights
etc.
Basically an extra heavy duty block design with heavier webbing, 4 bolt mains, drysump oiling etc.
SII heads were super trick HEAVY breathing design that could flow massive amounts of air.
Intakes were 4 barrel designs (now being converted to FI)
The heads were of such a design that (valves arranged: EI, EI, EI) SII setups using SII heads require a special cam and SII intake.
Most SII setups can be built to easilly produce 1200+ HP.