Make the 300 frame animation, DON'T RETIME IT, export it to After Effects and then export it from there at 60 fps and see what happens.

If you need the animation longer, you will have to go back and redo it and add more frames to the animation manually. Nothing you do with retiming or any other trick is going to give you the result you are looking for.

If I had a nickle for ever time a woman told me to get lost, I could buy Manhattan.