Not to bad. I did the passenger side I'm my 88. I even used one I cut out myself from the boneyard.
After I retrieved the piece from the junkyard:
I set it in place over the existing to figure out where it goes..
Then I cut the opening a little small so the they overlap just a bit. I then riveted them together pretty good to hold it in place. Then I welded the dog snot out of it. Then I used some silicon on any spots that looked like they could leak.
Just Becareful when cutting or drilling to watch out for fuel and brake lines.
I also primed and painted any bare metal to prevent rust.
I'm mot sure if the procedure is the best or is different when using a full brand new pan. But it worked great for me and you can't even tell.