malcala622-

To machine down the post is quite doable but you need to have it done at a machine shop. Because most seatposts have a set back for the saddle rails cradle, the machinist typically has to use a lathe with an eccentric chuck to hold the post. You might want to take the frame to a frame builder and have the seat tube reamed to handle the slightly larger diameter post. I have done the machine down but it was a real problem to find a shop willing to do the set up. I had a quote of $150 but eventually had a church member with a machine shop do it as a favour for nothing. The hand sanding approach should work as well but you should likely get the seat tube reamed anyway for a clean fit.