its the space between the flange and the block on the rwd motor that causes problems.
in rwd, the exhaust housing tucks under the rwd manifold close to the block.
using the rwd manifold on a fwd, has the exhaust housing on the radiator side of the manifold and that is the problem andre miko is trying to point out. you will also end up with the turbos compresor outlet where your radiator is.
have you really looked at how much room there is in the engine bay?