manny, i havent read this all the way through, so im not sure if someone suggested this or not, this is something that i have learned from hardcore racecar guys when you wanna drive a modified motor on the street with a holley...
I had a 377CI( destroked 400, a revving sob ill tell ya), in my 69 nova, i was having the SAME problems as you but my cam was a little bigger and i had a single plain intake, i was told to drill a hole in the throttle blades o nthe carb, not big just 1/32 or so, this leans out the idle circuit and makes it hold a idle better without turning the screw in a bunch... you see, with that cam it really should wanna idle around 900-1100, buy 1100 your out of the idle circuits in the carb and shes running fat...
i did this trick on my 750Double pumper and it was the best thing that i did.. no more dieseling, funky idle or loading up... i cant say this will cure your problem in whole because its a new motor and you will half to tweak on it for awhile to get it right, but this WILL help you if you decide to do it...
if anyone doesnt agree with me, let it be known but this worked for me and my plugs looked awesome after 1,000 miles and hitting the 7,500 RPM chip a few times.... just stay with the holley, once you have it tuned, you will be happy.. (although i would have went with a double pumper mechanical secondaries on it for how easy they are to tune, but the vacuum secondaries work fine too) hope this helps man.. otherwise, great sounding truck