Slant six dies in gear

Hey all, I’m new to this forum but my 1976 Plymouth duster restoration project has been having issues for the last few months and I’m getting desperate. I’m 18 years old and a first year heavy duty mechanic in college so I rely on her as my daily driver. Over the last few months she’s been having some overheating issues which resulted in me replacing the water pump, rad, thermostat etc to solve the problem. Just as I solved one issue another arose. I have a late 60’s slant six with the original transmission for my car (custom torque converter to make it work) and lately it’s been idling great in park and neutral but as soon as I go into drive or reverse the engine idle goes way down and dies. I feel it is worth noting that I have been running this engine and trans combo for just under two years with no issues. I currently have the rpm set at 950 rpm and 18 degrees. I’ve tried a new distributor as well as looked for vacuum and trans leaks. Have a new carb coming tomorrow but wanted to see if anyone else had any ideas.

Thanks in advance!