I think the question should be can I switch a 2.8 for a 3.8. Unless you have
air tools and don't mind changing out intake manifold gaskets, reshimming
starter motors and the other various tribulations a 3.1 will put you
through....
but, the only thing I'm not sure on is the engine mounts. The ECM and it's
entrails will have to be changed out because it won't be controlling a 2.8
anymore. Also keep in mind the 3.1's serpentine belt runs AROUND the upper
left engine mount.

Cheers,