Don I have a 408 that is similar to yours (just a tad larger cam) and at first I had a 770 Avenger on it and it ran real good but I did have to jet it up 2 sizes in front and 5 in the rear. As Rob said Holley's tech lacks a lot to be desired. I'd like to know why they said you had the wrong power valve in it cause if you've got 14 inches of vacuum that's plenty to hold a 8.5 PV shut. IMO the 8.5 is just about perfect for it. I'm only about 35 miles from you so if you have trouble getting it worked out let me know and I can take a spin down and have a look. Just PM me for my ph. # and we'll get hooked up.