I've been curious about exactly what the differences are in the exhaust port size and spacing between the 2.7 and the 3.5 so I went to the parts store and picked up exhaust gaskets for both. You can see from the photos that the spacing for the port on the 3.5 are a half inch further apart but the size of the ports are very close. So does that mean you can't fit them on a 2.7? not necessarily. By fabricating a plate about a half inch thick that angles the forward and rear ports out to the 3.5's and bolting it to the head then bolting the headers to it you could do it. There would be some loss due to the rather abrupt angle but it would certainly flow better than the stock exhaust.
Thought the stock 2nd gen exhaust manifolds sucked for flow due to the "U" turn the exhaust has to make in the manifold. That's why people have been putting 1st gen 3.5 manifolds on 2nd gen 3.5 engines to help in that area.