Terrible Casting...

I don't think you're necessarily in the minority, Nikon. A few on this thread have been a bit harsh on Huston - so some are going to be on her defensive, that's all.
Twenty years ago when it premiered, "The Grifters" was well reviewed and considered an acting coupe for each of it's leading stars. However, the physical dissimilarity between Huston and Bening was not lost either. In fact many reviewers pointed out that it was a distraction.

"Don't try to tell me that you looked at Huston and in any way thought her similar to the cute and spunky Bening?"
No I didn't. Bening's playful sexuality had a vigor that could barely be contained. She was a true sex kitten. A sharp contrast to Huston's icy, highfalutin broad. Does it make a difference? They're still very much alike in their views and motives.
As I said, I see the "issue" from all sides.

Here's the telling quote from Jim Thompson's book, I just googled it - it's Lilly from Roy's point of view. From this we can see that maybe the movie got some of it right, and maybe some of it wrong. Each be their own judge:

"She didn't seem to have aged a year in the seven since he'd last seen her. He was twenty-five, now, which meant that she was crowding thirty-nine. But she appeared to be in her very early thirties, say about thirty-one or -two. She looked like ...like... Why, of course! Moira Langtry! That was who she reminded him of. You couldn't say that they actually looked like each other; they were both brunettes and about the same size, but there was absolutely no facial resemblance. It was more a type similarity than a personal one. They were both members of the same flock; women who knew just what it took to preserve and enhance their natural attractiveness. Women who were either endowed with what it took, or spared no effort in getting it."