I got a 73 Duster slant six, automatic. It is sputtering and acting like it wants to die when it shifts into third gear, or when I floor it. Any ideas what it could be???

When did it start/any recent part changes. runs OK ex for if you floor it? check the easy stuff; AP squirt/filter/float level/vac leak then hook a can of gas to the pump inlet & set it in the eng compartment & take it out & see if it's a fuel delivery prob rearward from the tank

I just got it a month ago and got it running. Put a new fuel filter and starter in it. It drives great until I get on it and then it starts sputtering and acts like its gonna die. I will check that stuff and get back. Thanks!

Check the plugs/wires also.

1 BBL carb, or 2 bbl? I remember the 1 bbl's on my Dad's '67 valiant and '71 Swinger seemed to need frequent rebuilds because of stalling and hesitation.

might be sucking trash from tank. hard work stuffs the filter, less load lets it settle, and run normal. did you clean out the tank? try a see through filter, just long enough so you can see whats going on. some guys hate them, some love them. i put one inline, whenever i want to see what the fuels doing at the carb.