This is what I did (not because I could but because I had to :/ )
Long story short:
My 3.0 engine block failed due to one blocked cat. I got a used block from a scrapper, they said it is 3.0. Now this was a completely naked block, with the insides in, no accessories on it.
I then sat the two blocks next to each other and saw right away that they are different. Upon closer inspection I discovered that the new block is actually 3.2.
Since I had not time (and no money to spare) I slapped my 3.0 heads and accessories on the 3.2 block. So only the block and the insides of the block are 3.2, the rest is 3.0. Also put 3.2 head gaskets on since the 3.0 gaskets would be over the edge of the cylinder obviously.
The result: works just fine, with one little difference. When I did kickdown with the 3.0 block, the car changed gear right when it was about to hit the limiter. Now it actually lets the engine hit the limiter once or twice before it changes.