[Classic Mustang] Please help me

I have a '67 mustang and it has a straight 6 in it with a 200 engine and I was wondering why when it is driven for about 3o miles that it will cut out and pushing the gas pedal does not work. It is acting like it will die. I am a 23 year old female and I can not figure it out. I have changed the fuel pump (it was not getting a good enough fuel pressure) and spark plugs and the carb has just been rebuilt to "like new" condition. I noticed when I use fuel treatment and cleaner that it will run better for a little bit but after a while it starts cutting out again. Please help me I need to know I am trying to keep it "original" and I dont know what else to do please e mail me at blue_eyed_baby032002@yahoo.com to help me please I can answer a few questions about it if you need me to if it would help

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