'95 GSR 4-door, was 6yrs old & had 106k miles when I got the header. DC 4/2/1 ceramic-coated 2-piece header. No real problems installing it. I don't remember how long it took, but easily within a day.

Nuts/studs at the head & between manifold/A-pipe were OK.
O2 sensor isn't in this pipe anyway...
Nuts/bolts at the cat flange had to be broken/sawed apart, but I was prepared for that.

There's a bracket holding the original exhaust to the front of the block. That doesn't work with the header, I just left it off. It fit up OK with the bracket on the back. I had enough flex in the exhaust hangers, so I could push the cat around during this installation.