350 w/EGR system & slug heads?

Hi all!
Can you guys explain this to me?
What is an EGR system?
What are slug heads?
The motor is a 75-78 350 SB

Any info appreciated.........

Is everyone sick of seeing '57 chevy pics? If not I'll post some soon.
-Macklo

EGR is exhaust gas recirculation. "slug" heads would, I suppose, be junk heads or smogger large chamber heads.