So here's my 2 cents... I have a 76 Calais Sedan - my Old Man bought it new, and it's been with us ever since. I've been puttering around with this car since the early 2000's (about 20 years), and I've learned a few things. First, if you have an excellent near mint Quadrajet - send it out for a professional rebuild and add a 1" phenolic spacer ( Canton Racing 85-250 ), with the Fel Pro Marine Q-Jet gasket above and below (one above, one below the spacer) - the Fel Pro gasket is 0.23" thick, part number 17845. You'll need the thick gaskets and spacer to combat fuel boiling in the carb. This is an issue whether you use your stock carb or an aftermarket - it has to do with the Ethanol in modern fuel causing boiling at lower temperatures. It's a real PITA. The stock Q-Jet carbs were 800 - 820 CFM (yes, I said 800, look it up). Anyway - if your Q-Jet is beat to heck, has been manhandled over the years, worked on by an idiot, or is just plain gone, then an Edelbrock Performer 800 CFM ( part number 1413 ) is the way to go. "Oh no, that's too big!" -- lies, all lies. I am assuming that we are talking about STOCK OEM intake manifolds. If you have an aftermarket manifold, then it is likely single plane, and my comments are not applicable. However, if you have a STOCK OEM manifold, then it is DUAL PLANE (there is a divider wall between the left and right halves of the plenum, directly under the carb. With a dual plane manifold, any given cylinder only "sees" 2 barrels of the carburetor (one primary, and one secondary). None of the cylinders are ever exposed to all 4 barrels. In a single plane application, all 8 cylinders individually are exposed to all 4 barrels. Our stock Cadillac 472's and 500's have a dual plane cast iron manifold. Also, keep in mind the stock Q-Jet was 800-820 CFM. So, why would GM put such a big carb on a motor that only rev's to 4,000 or 4,500 RPM ? Partly because it's a dual plane manifold. It's a massive motor, and it needs to breathe. And why do so many people struggle with aftermarket carbs on these motors ? Because they need to breathe. You need a ton of air. The traditional metering springs / rods / and jets in most carburetors are going to be far too rich for the application - which is in part why so many vehicle owners will say that a 650 or 750 "runs good" out of the box (even when it doesn't). I ran a 650 for a while 15+ years ago, and it was TERRIBLE. Non responsive, sluggish, just a dog. The 800 Edelbrock would stall out from time to time, run pig-rich, and fouled plugs right out of the box. I dialed mine in with an Air-Fuel-Ratio gauge (wide band oxygen sensor), and these are the results... -- PARTS -- -- Edelbrock 1413 Performer Series Carb with Electric Choke -- -- Edelbrock 1480 Tuning Kit ( contains primary jets / rods / and metering springs ) -- -- Edelbrock 120-407 Primary Rods and 68-47 Primary Jets ( from the 1480 tuning kit ) -- -- Edelbrock PINK ( 66-187 ) Metering Springs ( from the 1480 tuning it ) -- -- Move Accelerator Pump Linkage 1 step leaner by moving linkage to the 2nd (MIDDLE) hole ( carb ships new with linkage in the 1st hole - closest to the carb body ). -- -- Edelbrock 1425 Secondary Jets ( 0.092" orifice ) to lean out wide open throttle ( these are purchased separately, not part of the tuning kit ) -- -- Choke is set to 1/8th open at cold, per Edelbrock manual. This is slightly tighter than the carb ships when new - it's about 1/4-inch open when shipped new. -- -- Edelbrock 77280 Throttle Kickdown Switch (they call it a 'Nitrous Kit', but all it does is close a switch when the throttle linkage is near 90+ percent, which I use to trigger the electronic transmission kickdown that is stock in my 76 Calais - the OEM switch was on the driver side and won't work with Holley or Edelbrock Linkage - only the stock Q-Jet linkage). Idle mixture screws will be roughly 1-1/4 turns out. Check this with an AFR gauge preferably, or dial it in for 20" of vacuum at 800 - 1,000 RPM. I use a 2" open element 14" round air cleaner for reference. Note - a 3" tall would be better, but won't clear my hood with the phenolic spacer installed. I usually let the car warm up for 10+ minutes when it is stone cold, or 3 to 5 minutes when coming out of the grocery store. The car runs like a scaled dog. I am exceedingly happy with the carburetor and the support guys at Edelbrock.