The supplier I plan on getting my 36 x 80 door from said the RO should
be 38 1/2 x 82 1/2.

I pulled the trim around the existing door and as best I can tell I
have 38 1/2 x 81 3/4. That's 3/4 too short.

If you look at this picture, you'll see that the plaster currently
comes all the way to the jamb. This is the side of the jamb, but the
same situation occurs at the top.

http://i440.photobucket.com/albums/q...14092135-2.jpg

As you can see from the picture, I chipped out some plaster to see
what's behind it. The answer is "nothing", meaning that there is a gap
between the jamb and the first 2 x 4. I have a half inch on both sides
of the jamb, so I have the full 38 1/2 width. That's good.

However, when I chipped out the plaster at the top of the door and
measured from the bottom of the header to the floor, I only have 81
3/4".

Top of door:

http://i440.photobucket.com/albums/q...1114092227.jpg

The current door slab measures 79 3/4" (not 80") and measures 81 1/2"
from the top of the jamb to the floor. Add a ~1/4" gap between the top
of the jamb and the header and I get ~81 3/4".

I should mention that I've had 2 estimates from local "door stores"
who took measurements, basically said "piece of cake" and gave me
quotes. They didn't pull any trim so they didn't measure the actual
rough opening.

Is 81 3/4" going to be tall enough or do I have header work to do?

Thanks!