I emailed Melling about all the old information online about the M55A oil pump and revieved this reply...
Yes, the M-55A is correct for the Z28. Standard volume high pressure bypass
Since 2008 the M-55A has had a robust HD casting.
Most info on the NET is BS.
Melling Engine Parts
Master Automobile Technician
(517) 787-8172 X email@example.com
I'm hoping this is correct. I am going to order it soon from either Summit or Jegs. If anyone has recently bought one please let me know what you think.