Z Tech Tips Z Exhaust Manifolds
Not much has changed over the years. The stock '74 and '81 exhaust manifolds are nearly identical except:
smog bolt holes at top of '74 runner (note pic has manifolds upside down) have been removed by '81. The later EFI manifolds have an EGR port instead (cropped out of photo).
the castings are the same dimensions however the 1981 ports have been opened up on the sides to make "D" and "Diamond" shapes. I am pretty sure this is a racer mod that made it's way back to the assembly line to improve performance and reduce exhaust gas reversion. It also accommodates the round exhaust ports.