Exhaust change. How bad will it be?

I'd have to agree.Your motor is a large air pump.Everything you've done was geared toward moving more air & fuel through the motor.So going backwards would really choke it down big time.Now you can still make plenty of power with stock type parts,but you build the motor with these parts in mind.With that huge cam & ported heads,you'll be bringing in in more air than the stock manifolds can extract.