Owning one of those cars, my advice is to leave the carby's alone, or at least leave to the very last. They can be a real problem to get in tune.
The clue that may help, is the fact that it stalls when in reverse, this is the only band in the transmission that can be adjusted.
These engines do not develope full power until they are at full operating temperature. Does this only do this when the car is first started, and go away when it is hot. So you may need to have the band adjustment checked.
Whats revs is the engine idleing at, if it is too low, then of course it will stall.
DO NOT fidle at this stage with anything other than the idle screw, the one screw can do both carbys together, it is located on the front of the front carby. You will see the throttle lever screw on the lever which opens the throttle. Idle should be 900 RPM with the motor at operating temperature. If you do not have a balancing device for the two carbies, leave it alone and find a shop that does have such a device.
This is not a job for the in experienced, and I will testify that if you play with anything else, the car will run worse, not better.
You need to get a service manual, Haynes make one 63020 346 referance and could be the best $20 you ever spent.