The duration needs to be derived from the openings and closings which should be listed in the engine manual (m116/m117 service manual, I believe seperate ones were made for the 3.8/5.0 and then the 4.2/5.6). There is also at least one posting on this forum that lists the openings, closings, and durations for common 3.5/4.5 cams; your best bet for power in a 420 aside from stock cams would probably be 3.5 cams with matching mechanical studs.