a manifold is typically a single piece of cast iron, while headers are made of a flange and 4 seperate pipes that collect into one. Header pipes have a less porus surface inside, allowing the gasses to flow easier, they are also all the same length i believe.

If and when you take the shield off, run the car for a minute to heat the manifold, expanding it slightly... this will make the bolts easier to remove, with less chance of breaking them off.

I painted my head shield in my 94 to match the valve cover... used the black "wrinkle" paint... came out really nice. Now when I remove the valve cover to adjust the valves, i'm gonna re-paint that too because there is some corosion starting. Also, replaced the 3 bolts with new stainless steel bolts, didnt want to ruin all that work with rusty old bolts.