Jerk and stall when put into gear?

The tranny slamming into drive sounds like the idle speed is too high. That could be due to a vacuum leak. My 69 225 used to run kind of like that. If idling lean, it tends to slow way down and miss when you shift to drive. Back to N and it starts to race past the idle speed you tried to set. Block off vacuum lines temporarily. If no help, try flowing propane from an unlit torch around the intake-block interface and under the carb. If it runs better, you have a leak. In my case, no gasket leaks. It was the many bad rebuilt carbs I tried. Finally found a good carb and it ran silky smooth.