[mikedsilva]

I'm far from an expert with these, but my personal experience has shown that our engines are VERY sensitive to exhaust and muffler changes.
I've also found that 'off the shelf' mufflers for modified engines, simply don't work well enough.

Pretty sure you'll need to have something custom fabbed up. What I would try first, is open pipes on the dyno, to see what the max potential of the motor is, then add your Monty back and retest.