Huh?
The Belvedere was the mid-priced Plymouth, and I really doubt if any Plymouth parts would fit a Pontiac.
Is it possible that you are referring to a Bonneville?
The Pontiac Executive–which was the new name for the old Pontiac Star Chief–was essentially a Bonneville w/o all of the bells & whistles.
For those who may have some old Bonneville parts lying around, the Executive model dates from the late '60s.