i had the same problem as you with my 408 using a 1000hp. no matter how much jet i put in it i would gain less than 1 mph or less and had no color on plugs. took it to the dyno and seen first hand what was going on. the air/fuel reading looked like the spiked lines on a heart scope,not even close to being a smooth line. i had a ngk7 in it and the dyno operator thought it was too hot and even an 8 to be too hot,so he put a champion 12 in it and the air/fuel reading had a nice smooth line. i'd try a colder plug just to see. my compression is 10.5:1.