For me, most of the times that I've installed headers, getting the old manifolds off was a bigger PITA than installing the Headers. I can think of a number of cars that getting Headers into the engine bay is a chore beyond belief. Most Fords with shock towers. I remember a 67 Fury that it took three men, loosening all motor mounts, dropping the starter and a mess of other things besides holding your tongue in the right position to just get the passenger side Header to even go into place, let alone putting any bolts or gaskets in.
The Pontiacs are not the toughest, but, that being said...I don't understand why people wouldn't use Header bolt locks when they install headers...Those bolts are going to loosen up after you get the Header's bolted in...Use locks and you will save time...and money, because you won't be replacing Header Gaskets nearly as often.