The MP correction factors for @.050 duration, as i recall, are 85% for aftermarket performance cams and 77.7% for stock cams.

Using the 77.7 factor for stock magnum cam gives:
268*.777 = 208
284*.777 = 220

This site seems to verify these numbers:
http://victorylibrary.com/mopar/mpcam-tech-c.htm