If you put 60fps footage into your timeline, it plays back at 30fps. That isn't conforming. It's skipping every other frame to keep the speed correct. Conforming is mapping every frame to a single frame in your timeline. To do that, click on the retime button at the upper rt of the project, and choose conform. Your clip will now be set to 50% speed and double in length. Sorry, but you'll just have to shorten the clip yourself. It doesn't maintain length. Easiest to simply not the duration at the bottom of the window before you conform. Then after conform, simply press ctrl+D (duration) and type the duration you noted.
It took FCP legacy about 10+ years to implement speed changes without changing the duration of the clip. Let's hope they do it a little quicker this go around. It's only been two. Give them time. :)