1) If the cam is indeed installed at 108 degrees, advance the cam four degrees. I wouldn't be surprised if you pick up low end torque AND top end power.

2) I think you are mistaken to believe the action plus manifold is junk. You do want to gasket match the roof of the runner exits because they neck down to 318 port size at the last inch or so. There is plenty of material to open them up to 340/360 size.

3) you could get a set of adjustable rockers and set the lifter preload to 1/8 turn or just were they stop making noise. (make sure you have good spring pressure) This will alow the engine to rev higher. You could try shifting at 6500.

4) I agree that a little bowl work on the heads goes a long way.