850 double pump issues HELP!!

Thats kinda a large carb for that engine, I would start at box stock settings, jets, squirters, pump cams etc. If you have the power valve plugs then yes you need to go up 4 jet sizes at the least for starters. I dont have a holley book with me here at the house so am wondering about the 50cc acc pumps, we generally just use those on alky burning engines, I use 35cc pumps on my race car and smaller squirters so it will squirt longer. If its a 4 corner idle carb then set all 4 idle screws at 1 1/8th turns out for a baseline. Usually a big stumble can be tweaked out easy, the most common stumble is just fine tuning with the transition from idle to WFO. Get some clear view sight plugs too, those are the cats *** and run the rear float level in the middle of the plug, accerleration pulls the fuel to the rear of the float bowl and the jet extensions help out there too. Another trick we use is just leave the power valve plug out and use the other side for extra fuel capacity, helps keep the carb a little cooler too.
And one last trick we do is to put the 850 base plate on a 750 carb, did that with mine and it flows @ 930 CFM, smaller venturis=better throttle response.