Carb is definetely NOT too large for the application, just needs to be set up properley.
I have found that anything more than about 1/3 fuel level above the sight hole will produce loading up & engine almost or stalling out under hard braking in my case. You can get away with it a little higher in some cases but most times it will flood under hard braking.
I suggested that as a starting point to eliminate that possibility. Easy to do & undo if no improvement.
A A/F meter goes a long way in sorting out these problems.
Craig
P.S.
I have a 920 on my 408---very streetable
My buddy has a 1000 on his 427W---also very streetable, watched it dynoed last night @ 452 RWHP
It can be done.