Holley tuning??

I'm having carb problems and was reading through the tech stuff and think I need a blow proof power valve. I was running a Edelbrock 750cfm and it ran fine. I bolted on this Holly 4340 double pumper and its running like crap. I shot the timing and have it at 8deg retarded but when I set the idle at around 1000 rpm it runs pretty good, but in gear stalls. It backfired out the top several times and is running super rich. I think I should order a power valve in case I blew it out but I'm not certain that's the proper fix. What timing should I run. It's a 360, victor 340 intake, headers, msd 6a ignition. It's my first Holly carb so any help would be pretty nice so I can get to Carlisle this weekend.