408 breaking up

It sounds lean though the cfm rating is not the problem but is a easy fix since with bigger carburetors comes the more radical metering blocks and such.

In nascar they sometimes run 390 cfm mech carbs and make 750+hp

If you give everyone here the carb modle# cfm, jetting, power valve/s it will help us improve the quality of advice you'll receive.

If it's a 750 dbl pumper try 73 primary and 80-82 secondary with matched power valve in front, plugged rear pv.
If you are running 2 pv's then square up the jets front and back, make them the same, and pv's should be the same as each other when running 2.

Sounds like lean jetting though.

Please explain the nascar thing to me,im intrigued on how thats possible...

An i hope cudapete,that your dart gets sorted,sounds like
she should be haulin way harder than 106 mph trap speed.