My dodge dart will not go past 70 mph at about 3000rpms?

I say its fuel related also, A bad coil will act'd up in any gear, its affected by RPM if its going out.

Not having full throttle on the carb will not cause it to break up & fall on its face.

I had this problem on my 73 swinger after installing a 440, strong through 1st. & most of 2nd. by 3rd. gear it was a wash, It was a fuel starvation problem, It drove me crazy trying different things, at idle the fuel supply all seemed fine, I dropped the tank & it was rusty & pick-up tube was 1/2 clogged, the main line to the engine was trash, it was rusted & a bunch of sludge/contamination, I was shocked it even ran. After installing all new line with 3/8 & a new gas tank it was a whole new ride & it hailed azz too.