You can do the rear main without removing the main cap, but once you've done all the work to get it all apart why wouldn't you want to replace the crucifix/cross seals?
What you want to do is pull the pan, remove the rear main cap (after removing the bolts holding it, put the pan bolts back in it a few threads and tap on the bolts to pop the cap out). Carefully clean both the grooves in the cap and the surfaces on the block. Put a small bead of right stuff in the grooves then press in the crucifix seals and (make sure the bearing is in place and put a little oil on it too) replace the rear main cap. Next you have to install the rear main, but you need a driver that will put even pressure all the way around and tap it on. I generally have a beer now and give the right stuff a min. to set up a bit. Now trim the crucifix seals even with the block using a razor blade. Reinstall the oil pan with right stuff. You especially want to make sure there is right stuff where the front cover meets the block and where the crucifix seals meet the pan.
If you have any questions you can PM me and I'll give you my cell.