Since you do not have the old cams specs, hard to speculate, but first thing I would do is do a compression test and get an idea of how much cylinder pressure you have. If you don't have 135 to 140 on a cranking compression test, I would advance the camshaft and try it again. The big drop in MPH tells me you lost HP somewhere. My experience is mopars like a like of ignition timing, 38 to 40 degrees is where most big blocks wind up on the dyno. And of course what do the plugs look like? Is your fuel system up to feeding your combo at 6,000rpm? These are the things I would check first.