The 2.9 pistons do fit in the 2.8 block. It's my engine....Everyone said they dont fit but if you look in the haynes manuel you will see that the bores are the same size the difference is in the stroke. The 2.9 pistons have a far deeper bowl that the 2.8 which helped to lower the compression.

I even have a build thread on it for when we bought the 2.9 engine and swapped out the pistons.