Chevrolet: Engine dies when I shift tranny in gear., cicuits, carbs

Question
I had tranny rebilt by ammco transmission and they say linkage is set right. when car is put in gear it tries to go back or forwaard but dies immediately. I have check idle cicuits in carbs and every thing appears normal. But even when car is in park idle has to be set at 1200rpm or higher or engine will die.But running two carbs on motor with perfomance cam this would be normal I would think. At this point I'm not sure what to look for next. Unless Ammco is wrong and not wanting to go back in tranny to find whatever maybe causing problem. If you can think of anything else that may cause motor to die when put under load please let me know.

Answer
HI;
When You say 2 carbs and a cam- did you ever change them on that engine ? is that why it has to be set at 1200 rpms to idle. if you have a big cam in it that could be part of what is wrong but if all is stock I still think it is the tranny> as if the tranny is not working right it can send pressures to places it should not and if in two gears at once will kill the engine so I say if the engine is stock with the carb you now have or with 2 carbs it should not die out, what carb do you have , I would try a diff. carb on it to be safe.
let me know what you find ok
roger