4 barrel carb

I'm thinking of going to a small 4 barrel carb. What would be the brands to stay away from and what would be the ones to consider? I have a 258 bored 30 over (makes it a 261 I believe) with a Comp 4x4 cam.

I was looking at the Edelbrock Thunder series AVS Off Road carb (650 cfm)and the Edelbrock Performer Series carb (600 cjm).

Those were the ones in the first magazine I grabbed anyways. I've heard Holleys don't run well at angles and they are real sensitive to getting dirt.

What is the calculation for figuring how much cfm you need? Does anyone make a 500 cfm? I'm wondering if 600 cfm is too big. It says they are good for a small block V8 which is what the MC2100 I have came from.

I was thinking the Motorcraft might be too small. I had rejetted it once and saw a slight performance increase but also saw the economy decrease. Someone told me that a small 4 barrel might get me better economy because during cruising you would only be running on the smaller barrels and the secondaries would only open up during accellerations. I was looking at a 500 cfm Quadrajet or Edelbroc with the "off road" package. I have a few other things to try first. There are just some things with the Motorcraft I have that bother me. I never could pull more than 17 in Hg of vacuum with this motor and now I'm bouncing between 17 and 15. I'm confident I don't have a vacuum leak around the intake but am going to check again. A friend thinks the vacuum advance diaphram might have a slight leak but when I suck down on the hose to move the lever inside, it stays in position.

I think I can hear a slight whistle when driving but it won't make the sound while idling. I replaced the vacuum hoses and the EGR valve (even though it isn't hooked up it does have a diaphram hooked up to vacuum).