I wish I had known yesterday Doug, I was at my storage building, I could have checked my '66 7 Litre Galaxie for an exact measurement. My '66 7 Litre 428 has the original C6AE-G "S" cast iron intake manifold, I have a 390 GT Holley on it with the 289 "HiPo" style reproduction chrome air cleaner, which is a bit of a drop base if I remember. I don't remember if I have any kind of spacer, if I do no more than 1/2". It clears the hood obviously, but I don't remember how much extra room there is, I'd guess at least 1-2", but just a guess. If you have Jay's intake book it shows the measurements of the intakes, the Performer RPM isn't too much higher than the "S", a little more than an inch higher at the front of the intake, but only a quarter inch higher at the back, and the carb is set back more which should help. Unless someone can get you the measurement I'd probably go with the drop base, my experience has been the taller the element you can run the better.