To help with your search the correct oil pump & pick up for the 3879633 deep oil pan is the #3964255
Reference: GMPARTSWIKI lists the following description for the associated part numbers.
#3964255 Pump ASSY. (Oil) (Deep Oil pan Only)
#3946023 Pass. 396, Pump ASSY Oil
Also, see the attached posts:
https://www.yenko.net/forum/showthread.php?t=152631
https://www.yenko.net/forum/showthread.php?t=151622
I hope this helps.