holley 4150 carb issues and questions

carb is a 850cfm on a mild 440 with a 727 trans.
the history of carb and engine are as followed.
68 440hp
.20 over flat top pistons
mopar "purple cam"
crane push rods,springs and adjustable rockers
torker manifold
mopar distributor
p/p stock heads

engine was rebuilt @ 20 years ago. was put in car drove around a few miles(less then 1000) then parted.
engine/trans sat in heated garage till I bought it a few months ago.

being that it sat so long I rebuilt carb. I'm thinking I may just need a carb or I looked over something as it was kind of crappy in side. I soaked it a few days in a carb cleaner bucket, blew out passages with air and put it back together(with a renewal kit). the issue im having is a part throttle stumble say %15-20 throttle just off idle. I've adjusted acc pumps and can see it pump fuel threw out throttle travel. its drives fine once past that one spot

things I checked
fuel pressure 6 psi
timing @13-15 deg advanced totals out @ 32
new ballast resistor

im thinking of replacing with a preform or other holley based carb.

what do you guys recommend for a street car?
71 duster 440/727 2500stall with 323 rear gear
mech or vac secondary 750 or 850

thanks