If the headers are made to fit a g body they should go in easily. Usually the drivers side you can get the header in from the top. Just the AC and power steering brackets need removed and modify a little to work and add a few spacers to take up the space of the manifold. The passenger side usually goes on from the bottom. And a 3 wire (heated) O2 sensor is recommended as stated above.
1984 SS, w/87 Grand National 2004R, 355 vortec, CCC, Crane XE262.
1969 Road Runner
2003 GMC Serria