I found some LT4 Rockers,Seats, Retainers and springs for a good deal, will these do much for my car?

and what came would be best to be used? hot cam?

 

I see a 503 cam for sale in the sale section, could that be safely run?

 

TRUE LT4 rockers are for a metric stud.
The LT4 springs are about right for a stock cam with 1.6 rockers despite whatever GM and the young and inexperianced here will tell you they are NOT enough for the HOT cam. after a fairly short time they will lose pressure and become insufficient for the HOT. The LT4 used lighter valves than the LT1 which is a piece of why GM got away with such a weak spring.