probably carb problems

Hello all, I'll start with thanking you for taking time to read and reply to this. Now, I have a 1968 Mustang with a 302 and a 625cfm demon
road jr carb, a realatively mild comp cams, (I think 168 degrees duration and something like .460 inches of lift), the engine is presently set at about 16 degrees advance, (the cam manufacturer recommended 14-20, I believe) and I have tried anywhere from 10 degrees retarded to 30 degrees advanced, and am still having this problem. Now, the problem is that the engine runs GREAT if the transmission is in neutral or park (it is a c-4 that was JUST rebuilt) but as soon as its shifted into gear the rpms drop (as they should, I think) and the engine dies if i dont give it gas right away, and it doesnt pull at all under acceleration. The engine pulls anywhere from 5-15 inches of vaccum, depending on timing, I get the best vaccum the more advanced it is. It has power breaks, and the breaks are really "soft" which leads me to believe that there is a vaccum leak...but would this cause this problem, even if I can get 15 inches of vaccum? Any info or help or suggestions would be awesome! Thanks again for your time!
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