Engine stall.

Hi I have a question and am looking for any ideas how to fix my problem. I have a rebuilt 355 in my 1968 Chevelle. It runs great, but when I am in neutral or park and put it into gear it will stall unless I really give it a lot of throttle. If I start it in gear it will go straight to a nice 1000 RPM idle. I am drawing vacuum directly from the manifold for the trans. and from the carb base plate for the vacuum advance. It has manual brakes. The engine has RPM intake, 650 double pumper carb, .488 lift 274 duration solid lifter cam, 9:1 compression, with World S/R torquer heads. The trans is a rebuilt TH400 with a 2200 stall converter. I assume this is a vacuum issue but not sure any ideas on how to fix this will be appreciated. Thanks Jay.