What size carburetor should I be using LA 360 engine?

I have a 360 in my 68 Dart with a Holley 750 on it and for a while I had the same "issue" as you with the car running great in Park, then wanting to die the second I put it in gear. What I ended up doing was buying an engine analyzer off CL ($10) and finding an old timing light ($10) and just started turning things. In the end my timing was WAY off, and my floats were also WAY off. I am by no means an expert and I am still learning but I've gotten to the point where I can get it idling good now. If I had to say anything about the carb size, I would say that if anything, it might be too big for normal day to day driving but then again, my biggest assumption now with the car is that if something isnt running right, then I haven't adjusted something correctly