I have to both disappoint and comfort you Steve.

The great hair you see in the movie IS in FACT fake. Pretty good one though. I am absolutely positive of it. I made a "research" a while ago and all I found was a bunch of pictures of the actress with less then shoulder length hair. The fact is she never had her hair this long.

Actually, you can see this in the movie as well. You can always see this no matter how well the extensions are done. The sorter hair is strangely "layered" over the longer one. It is less noticeable in the black and white movies.

Here is another example:

The movie "Lenny" (1974) with Dustin Hoffman and Valerie Perrine where the latter (obviously) plays an extremely long-haired stripper.

Valerie Perrine never had her hair this long and in one seen where she is dancing in a club you see her real hair "goes apart" from the extension in a flip. It looks very natural though when she doesn't move.