What year is it? They changed the rear main seal to 1-piece in '87, but either is fairly straightforward... the downside is you need to drop the pan to do either. The 1-piece seal is easier to replace because you don't have to remove the main cap, the 2-piece you do. The 1-piece you drop the pan, remove an aluminum retainer and change the seal, unfortunately the flywheel/flexplate has to come off for this. The 2-piece is slightly more involved... since your trans is out I'd say go ahead and remove the flywheel either way, easier to work around.