A stock SBC valve spring has about 80-90#'s on the seat and 240#'s open.
If you look hard enough there are springs that will give you over 200# on the seat and over 400# open and will fit into a stock package if setup correctly.
Not recommended for daily drivers but for class racing it's pretty common.
I'm not quite sure how this thread got so off topic. The reality is while it's easier to get higher pressures with bigger diameter springs, the statement you made about there being no stock diameter springs that can give you 350# open is just not factual in this day and age.
PAC-1409 Stock Eliminator Beehive Springs