79 cougar stalls when put into gear?

It was running ok but now the carb is backfiring I'm going to try to put a new carb on and see how that goes we replaced the transmission modulator as that was bad so I can see about draining tranny fluid before the new carb thank you so much for the input by the way I'll keep you posted

I would replace the idol control valve. A vacuum leak could be your culprit. They can be a real challenge to find. I've had them underneath on a elbow thats made bigger on one end than the other. I wish I knew what its called but I don't. Good luck