It was easy. The O2 sensor hates being removed, so use some O2-safe antiseize on the threads when you install. The spring bolts can be overtightened; you want to get them snug enough to make a seal, but still able to flex because that's what these cars use instead of the flex-pipe.
You can install the header from the top, but I'm not sure the old one can come out the top or not. Just maneuver it over the transmission. The 2ZZ is a different story, so be relieved you don't need to do that (those manifolds are actually headers and only need to be port-matched anyway).
"If you don't love me at my diddliest, you don't deserve me at my doodliest." - User ktluvscricket of reddit
Well, ive been spraying all the header bolts with AC delco rust penetrator daily to make it easier when loosening the bolts. I have the header and i just got the heat shield off the old one which was not that easy. Should I look into replacing all of the bolts in the assembly or should i be ok sticking with the factory bolts?