It isn't a day at the beach, but yes... headers can be swapped on an E-Body without removing the engine. I'm on my third set. Don't ask...
The driver's side is always the fun part. I had to pull back the steering column, and disconnect the kick-down linkage(I have an auto). In my case, the starter and header had to come out at the same time, as the starter was encaged in the Hedman's I have. I have a 440, but I'm sure your results will be similar. Also be aware that some of your header bolts might go thru the water jacket. In that case, you will need some thread sealer to re-install them.
Mike