The RPK 109 remodeler kit includes two RP 135009 plates and one CV-86 plate. Using this kit allows you to convert a standard 8" mortise lock prep to a cylindrical lock prep.

You will still need to drill holes for the knobs, typically 2-1/8 holes, but this kit might just save the day. 

the two RPK Plates measure at 9" by 3-1/2" and the cv-86 measures at 8" by 1-1/4".  Available in 630 Brushed stainless or 605 polished brass.