I know the firewall on my 85 had to be slightly modified on the passenger side to fit the manifold but I have a Cleveland so if that's what your friend has I think that's the only thing you have to look out for fitment wise. I'm not sure about mechanical problems right off hand although my car does have a push button start so the ignition might be something to look out for.
~If speed kills then buy a Honda~
~I'm not addicted to gambling... I'm addicted to sitting in a semi-circle~
83 Thunderbird with a 351 Cleveland