Originally Posted by potus
it'll probably fit with-out cutting. try and see. as long as you don't force it and use plenty of grease you're not going to damage anything by trying.
+1
Try applying grease inside the seat tube before you insert the post, instead of simply applying grease to the post itself. I had a super-long MTB post on my previous bike that probably went 6-7" into the frame. Even though I greased the post, I think a lot rubbed off during insertion. After several weeks of wet riding, the post was cemented into the frame. When I finally extracted it (no small feat), it was the very end of the post that had corroded to the frame.