the old stem was probably hard to remove due to corrosion, not because it was the wrong size. if it fit in there, that's the right size. start by cleaning out the seat tube, make sure to remove any old crud and rust. then get a new 27.2 seatpost and make sure to apply grease to the seatpost and inside of the seat tube.

if you use a smaller seatpost, it will be prone to shifting under weight and can also damage the seatpost clamp.