It was a Rochester Quadrajet on it originally. My '68 Corvette had this set up.
They're good carburetors until they wear out. If you want to stay with that, Summit Racing sells rebuilt Q-jets with 3 levels of performance. They range from about $330 to $390 bucks.
Holley used to carry a spreadbore carburetor, which my 327 was switched to. Much easier to tune, but the secondary bores are not nearly as huge as the Rochesters. I remember Q-jets. You really felt them when they went into the secondaries. Kind of felt like an afterburner.
Another thing you could do is go to a modern intake manifold. An excellent choice is the Edelbrock RPM Air Gap. It is so good, I switched to that on both my 68 Firebird and my 68 Corvette. The difference in power is very noticeable, and will allow you to use a standard square bore carb.
The formula for carb sizing is:
CFM= Displacement x redline RPM x Volumetric efficiency / 3456
Volumetric efficiency is about 0.90 for a good street motor. Race motors approach 1.00. Turbo/supercharged motors are over 1.00.
So for a stock 300 horse 327 with about a 5500 rpm redline, here's your goal:
CFM = 327 x 5500 x 0.9/3456 = 468 CFM
So you really don't want a real huge carb. Since you can't get a 468 cfm carb, you can step up to a 600 cfm maybe. The closer you are to your ideal cfm range, the easier it will be to tune and the more responsive the engine will be. This is important for a double pumper especially.
Vacuum secondary carbs can be bigger, because the engine will determine how far the secondaries open. So you can go larger with a vacuum secondary carb. A 750 cfm in this case would not be a problem. Vacuum secondary carbs can be used in manual tranny cars, but are usually used with automatic transmissions.
