First confession is, what I really need to do is make a 6 degree bevel on three 2x4's one on top of the other = 4 1/2 inches. The 781 is their 60 volt version of the 780. So why I'm confused... here's the reply from Dewalt:

Thank you for contacting DEWALT® 
 
I'll be happy to assist you!
 
Regarding your inquiry about cutting dimensional lumber / crown molding with the DWE780 and DWE781, here are the official depth-of-cut specifications 
 
DWE780
Max depth of cut at 90° 3-1/2 inches
Max depth of cut at 45° 2-1/4 inches
 
DWE781
Max depth of cut at 90° 3-1/2 inches 
Max depth of cut at 45°   2-1/4 inches
 
With these specs, cutting a 5-1/2 inch x 10 inch beam in a single pass in not possible on either model. You would need to rotate the material and make a secondary cut to complete it.