I once accidently put my MSD ignition on 6 cyl and I didn't notice the problem until I staged the car and hit the throttle. Way down on power and didn't sound/feel right, aborted run. i couldn't believe it wasn't more noticable.

So triple check the wires cut to make sure you are set for 8 cyl.

I have also seen cams ground way off, maybe 8 degrees as I recall. This was using the same crank, timing set, and block setup that I checked 5 other cams on. So it was the cam, the degree wheel didn't lie.

And if the polarity of the MSD connection to the distributer pickup is wrong, it will run like crap also. I have even seen the wire color code wrong. Swaping polarity did the trick.