I have these on my 79. I'm probably repeating some of what was already said....BUT...I had to do some trimming to the rocker panels. The headers come with a sheet that tells you the measurements for trimming the panels. Follow those and you will be golden. I have the ceramic headers and side pipes..ordered from Summit...no regrets.
As far as what needs to be removed...I have a manual...had to remove the oil dip-stick, clutch assembly (the push-rod that attaches to the block), and the oil pressure sending unit. Nothing too major. Fishing the headers up under the car will be a challenge. Took me and a buddy the better part of 6 hours to complete the job.
As far as mufflers...i ran open headers for about 5 months. after I failed my hearing test
i finally went with the hooker max-flow inserts. I would highly recommend them. Not too loud, comfortable to drive, but still mean as hell when you stand on the throttle.
Finally...i went with a copper gasket and have the stage 8 header bolts to prevent them from backing out.
Hope this helps, and sorry if i repeated info from other posts.
good luck with everything.
-Shane