the only problem with using the 390 holley's on the edelbrock dual quad intake is that the intake was only designed to use the edelbrock/carter style carbs. the holley's wont fit. one of the ford mags put an edelbrock dual quad intake with two 500's on a stock 84 mustang 5.0 a number of years ago, and reported that the settup was very good overall. the progressive linkage was smooth, throttle response was good, and fuel economy was pretty good also. i dont recall what the downside was, but it was insignificant.