Engine shakes BADLY!!

Wow thanks guys! Sorry I have been away, Xmas getting in the way. Anyways I'll try and up date you guys on what's up..
Higher initial is WAY better for this 340' when had it down below 17* it wouldn't idle, so she wants the advance. Crackedback is right , if I advance it , the idle does go up. Right now as I said earlier i have it at 19-20* initial. Witch puts me at around 38 total. Does msd make a different bushing to get that total down even more. Obiously I put in the black one to give me the shortest, but that's 18*,where I'd probally be better off around 14*.
Anyways on to newer problems, trying to dial this engine in. Me and my dad ( who many of you know, from here) have been saying this carb was built on a Friday from day one. I am not a carburetor genus but I'm getting better as this project goes on. I wanted to make sure that this carb didn't come with 2 power valves. So today I had some free time and decided to take it off, take it apart and see if I could find anything wrong. Took it apart today , put it back together, started the engine up and noticed in drive at 750rpm had only around 3" of vacuum.
So I bumped up my curb idle to 900 and was able to get the vacuum around 5".
Now, I revved the engine at got a carb backfire...hmmmmm, only did it once or twice but now I have a hesitation when going from idle to a snap of the throttle. (in park).

Here some details of the carb and let me know if you thing anything sounds out of place..
Holley 750 d/p no choke.
Pink accelerator pump cam @ number 1 position. Same with secondary.
Squirter on is a 28 ( to small??) and 31 secondaries.
I have my idle screws set at 1 turn out.
Not sure what jets rights now.

I checked plugs last week , and I'm getting a fouled dry carbon color. I'm thinking my plugs are too cold of a temp. There ngk br7s I believe. Not 100% sure right now, flyfish has the same plugs. Could this be part of my problem?.???


I know I'm going on like a story here but just thought the more info for you guys , the better you can help me out.
Still getting shake from engine, still being a paint to start one warm (free spin) until I pump the gas and Matt and get it to spark.

Thanks for all the compliments guys. Thanks for all the help. Merry Christmas