Functionality has been improved since the 6x series, but the architecture works a bit differently.
While previewing a clip, right-click the preview screen and set Widescreen A/R Fit to Letterbox.
On the cut-off clips, open effects and try applying one of the three choices in the Aspect Ratio Transforms section. Letterbox is the likely choice.
In export leave the A/R settings as suggested.
If this does not achieve the desired result, share the project and someone will come up with a solution. It's easy, takes but minutes and can be private.
Here's how... http://nch.invisionzone.com/topic/23659-tips-for-getting-help-on-this-forum/
If it contains sensitive material link it in a personal message (PM). Click the envelope above. It won't be passed on or retained. Use FILE|SAVE PORTABLE PROJECT AS, not simply SAVE PROJECT, and don't forget to get a shareable link to post.