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it depends a lot on how optimised your converter is for your combination. in a previous life i experimented with gear ratios, and found that the converter is way more important than getting the theoretical optimum gear ratio.

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Switching from 3.91 to 3.23 will make the converter work harder, and if it is too tight, it could prevent the motor from getting up into the power band quickly. So how much time you lose will depend a lot on the converter.

Another thing to think about, the switch to the 3:23 gear will place a higher torque load on the gear set, increasing the chances of breaking the bearing caps. (think of it like a 10-speed bike - attempting to start out fast in a low gear just gets your feet moving fast. Start in a high gear and you can feel the pedal arms flex, the frame creak and groan.)

With 550hp you might want to start thinking about a Dana. Yes, I know this has been hashed out over and over on this board, with lots of poeple saying they never have problems with a 8-3/4. But most of those folks either have light cars, less than 500hp (despite what they think they have), or never run sticky tires.

Back in the 80's I never thought I would need a Dana either, that was before I knew what actual horsepower and traction were. After several hogs heads, twisted axles, and bent housings, it became evident switching to a Dana was a bargain. Oh yeah, that was in a 3800# '71 SuperBee running 11.70's (slicks and 4-speed).

Scott