Following this thread as I just installed an 800 Thunder on my 440. Right out of the box I had much better throttle response over my 750 vac sec Holley.
I am not experiencing any off idle stumbles at all.

Only thing I am battling is a "dirty" idle. Has the same eye watering, clothes stinking smell as my Holley did.
Only installed it Monday & really didnt do any tuning to it except try & play with mixture screws (didnt really help)
Looking forward to more of your updates!