1969 Chevy Camaro hp dropping by at least 100

I own a 1969 chevrolet camaro z28 that has a digital performance monitor on the dash that works perfectly the horsepower on this vehicle will drop from 462 hp to 310 hp every ten miles i need to know what could cause this because i just put a brand new smallblock chevy v8 straight from the factory in it and the only thing i did was put an intercooler on it as well as a supercharger from hennessey