I had this same issue on my 340 with an 800 cfm Edelbrock
Beat my head against the wall for dam near a year floats, rods, springs, settings, jets. Finally figured it out the old rule of thumb to set the mixture screws for highest idle and then go a 1/4 turn rich is a bad practice - stalled under hard turning and breaking everytime.
Now I set the screws for highest vacuum and the go a 1/4-1/3 turn lean. no issues
