[StormBringer]

I was going to say the same thing. Are you sure that you are not seeing boost creep on the gauge. It also might be in your interest to install a temp sensor inside the intake housing after the blower.

Also since this is a blow through carb design you might need to get a larger carb. The ventri's might be to small for the amount of boost that you are running, or you have a head restriction. You should see a lot higher numbers then that. On my SB 350 I am approching ~550 on 5-6 pounds of boost at the crank, mind you I run a different style of blower.