What are the head codes from the center exhaust port? 140, 141 etc. would be 326 heads... which would have tipped off your Mechanic. He didn't tell you what lead him to believe it was not a 400 motor? Being an old Pontiac guy, I would think he would explain this?

Being an old Mechanic... he could be getting things confused? I hope for your sake!

Here is a list of casting numbers that may help in the mean time?
http://www.teufert.net/pontiac/bloccast.htm
http://www.teufert.net/identify/identify.htm

Keep us posted!
Brett - 67 Drop Top 455!