Chevy 305 idles great in park but very low & rough in gear?

[Rust-O-Matic]

As the title sugguests, the '78 305 small block in my '66 C20 idles well but the rpms drop when I put it in gear, making it shake and shudder like it's going to stall. I have messed with it quite a bit, read countless threads on google, this site, and other forums, and finally just decided to ask what you guys think. So, here's some info:

Engine and tranny were already in the truck when I pulled it out of a field and brought it back to life. Stock 305, Edelbrock 1405 carb (on it when I got the truck), Hooker 2466HKR headers, hei, new plugs, new wires, new cap/rotor. Trans is a TH400 auto.

I just recently got exhaust pipes put on, so now that I can actually hear the engine, I decided check timing and properly adjust the carb. Got my timing light out, only to realize this engine is supposed to have the 12 o clock timing tab on the chain cover, but someone has put the other type (like a normal 350) that doesn't have that tab and instead has the tab over on the driver side. So the mark on my balancer is obviously nowhere near the timing tab on the chain cover. Basically all I could do with the timing light was verify the advance was working, because I could see the timing mark move when I gave it gas.

So, I ended up just advancing the distributor until idle quit speeding up/improving, backed it off a little and tightened it back up.

Then I adjusted the carb mixture screws on the front by turning them all the way in, backing them out 1.5 turns, starting the engine, backing the right one out until the engine quit speeding up/smoothing out, then turned it back in like 1/8 turn. Then I did the same to the left screw. Then I adjusted the idle speed to a reasonable level (no tach available) via the screw where the linkage attaches to the carb.

Sorry for the windy description, but I want to be specific. Ok, so now the thing idles really good in park, but when I put it in gear the rpms drop real low and the truck shakes all over the place til I give it some throttle. It actually died at one point. So I turned the idle speed up a little. Still same problem. So I turned the idle speed WAY up just to see what happens, STILL same problem, plus it really slammed into gear. So I turned the idle back down where I started.

Next, I checked for vaccum leaks all around the carb, vacuum advance line, and where the transmission hose attaches with a spray bottle of water. I didn't notice any leakage whatsoever.

So now I'm scratching my head wondering what to do next. Is my timing wrong? Carb gunked up? The damn thing runs great down the highway, just idles way low at stop signs.

I would like to set the timing "correctly" with my light and get a tach just so I can see actual idle rpm. What's the best way to check the timing with the incorrect chain cover I have? Could I attach the timing light to a different plug wire so the flash would happen when the mark on the balancer is lined up with the 350 style timing tab?

Any input or suggestions would be much appreciated.