Issue with stalling when in gear

So I recently bought a 1967 barracuda convertible. I’ve had issues with when putting in gear it stalls. Also, I had the transmission rebuilt with a new torque converter. At a stop light. I have to hold the brake and give it gas but the engine still seems to pull forward until it stalls. I am at a loss. It has a 904 transmission with a slant 6 225. I am about to give up on it. Any thoughts on what the problem is. It had the same issue before rebuilding the transmission. Thanks for any suggestions.