Video preview when working in non-standard video format

Hey Guys, 

I don't know if this has been discussed before but I could not find anything.

I'm working on a project in which the final video has to be 3456*1080px. I have a BMD UltraStudioMini 4k outputting via 12G-SDI to a 4K Monitor for preview. Now, whenever I work in a non-standard video format Premiere gives me a more or less unusable preview. In this Case 2048*1080 which results in a squeezed image. Is there any way to force PP or the UltraStudio to center the 3456*1080px timeline in a regular UHD. 

THX