Heat the frame, freeze the cone.
Even boiling water can help on the frame if you don't have an oxy or don't want to damage paint with flame.
Sand the seat and get it smooth.
A socket a tad smaller than the cone and a 6 inch extension so you don't hit your fingers will help to get the cone on the right angle so you can tap it in, won't go in if its not right on.
Geoff