Fantastic idle in park/neutral, runs terribly/does not run in drive

[Peterman]

I have a 74 which I recently put an l16 into after the death of my l18. The engine was running fine, but I started getting a clogged idle system on my weber. It was rust in my tank- fixed now, sealed up with por-15. Now I have a new problem. The truck starts up and idles flawlessly in park or neutral. Put it in gear and the idle is rough. Sometimes it is rough overall, sometimes I can get it to run fairly well, but it stutters or misses every 5 seconds or so. Everything is tight, vacuum lines in place/blocked off, and I've tried adjusting the idle and air mixture screws all over the board with no success. Does this sound like a carb rebuild is necessary, or maybe something else?

[datzenmike]

When driven does it up shift alright? If not find the vacuum line that goes down to the modulator valve from the intake. Make sure it was put back on when the engine was put in. It is likely found on the fitting the brake booster hose fits onto. 

[Peterman]

I was hoping the modulator valve wasn't connected as you described, but it was. I put new hose on, messed with the points a little and it's at least drivable now. Still idles pretty rough though. Looking at the vacuum modulator, it is in fact connected to the same fitting that runs to the brake booster from the intake. My intake has a second barb (not the egr, but right next to it) that I can't figure out where to connect, but saw someone had connected this to their vacuum modulator line. I currently have it capped. T it to the vacuum modulator or leave it? Sometimes this truck seems to decide randomly how it wants to run :(

[datzenmike]

How is the idle in gear?