That's a great engine combo. I have some info that may be helpful... When we dynoed my 408 with very similar specs, I was disappointed with the performer rpm. If memory serves it picked up somewhere around 20 hp and 15 ft lbs (observed) across the board with a M1 single plane. Now, we did start the pulls at 3500 rpm,and both were done with a 1 inch Wilson spacer and a 1000cfm 4150 on E85. I don't know what it would have done at lower rpms with a smaller carb, but it peaked with 578hp at 6200, even with the M1...which was the same peak rpm as the performer rpm. Maybe I'm comparing apples to oranges? Keep in mind it was dynoed at 6300 ft, and according to my engine builder it should have liked the performer rpm better.(which is why we started out with it)