I couldn't find a New Jersey will abstract for any William Webster 1670-1785, though he/they witnessed a few wills.
There are some land records but men with the name, but I don't think they are for them.
Shotwell's Annals page 12 lists no DOD for William II, with William III on page 13 born 27d 7m 1723, married to Mary Thorn in 1749, dying in 1750.
I would look in the ratables (tax) index for them, but the Library is still closed.
You might want to have a look at some of the county histories listed on Sources-New Jersey.
Sidebar--Ancestry is a pain for some of these early New Jersey Quaker meetings, the only records you can find are compilations of dates compiled long after the fact (sometimes 150 years later), like the ones listed in other answers.