Before you go reaming it if the seat post is being gouged like a cutter is scribing it then check around the split area where the seatpost clamp goes. I've seen some bad edges in there that need to be dressed back with a file or something like a Dremel with a ball grinding bit. Some work at the top tube clamping split with this and you may find that it goes in just fine. After the file or grinding some fine sandpaper by hand to polish it a little can't hurt either.

I'll bet the post slips in just fine after that.