If you're using the stock pulleys the blower speed will be the same, but the boost may not match. Your cam, ports and valves don't match the 3.8, your 250 is slightly larger than the 231" 3800, which may change the boost level, as will how big your manifold plenum is.
Again: that valve is vacuum-sensitive, when it closes doesn't depend on throttle opening or RPM but on falling vacuum, which is affected by gearing, chassis weight, exhaust backpressure - long list. I assume this a draw-through with a carb adapted to the Eaton intake? How big?
