serious carb issues

hey guys,

im having some pretty big problems with a carb that i had made for me. Its a 750hp carb and i have it on top of a mild 340 mopar engine.

My problem is getting the car to stay alive. Before the lowest i could get it to idle was 1200 (much too high for a street car...with the old carb it idled down by 700) so the guy i bought it from told me to close the secondaries all the way at idle, which has seemed to help the problem...However, now even when warm the car wants to die, especially after i rev it up, it just stumbles like crazy and shuts down unless i throttle it...When adjusting the 4 corner idle, unless i have the driver side primary screw turned in all the way, the stumbling gets worse...as of right now the transfer slot is not exposed, and i was told by the builder that in order to fix this problem i would have to drill bigger holes in the blades until the problem stops and i get correct idle...i dont know how well this solution would work and id rather not drill any holes

The timing is set at 15 initial, 36 total, with 13 inches of vacuum..The car is also backfiring quite a bit (thinking i might have bad valves, but im hoping its just a carb issue)...its got 10.5:1 pistons, but im running 89 octane and the cam specs out 230 duration at .50, 480 lift (single pattern)...There is no pinging, but im not sure if the backfire is a sign of the gas not being high enough octance...

Any help anyone can provide i would really appreciate...the car is running like a total bag of crap right now, and its pissing me off since this carb was supposed to be a set the idle and go kinda deal (though nothing really ever is in this hobby).

thanks!
Matthew