Too much pressure??

1. Needle/seat issues including junk in it not letting it seat.
2. Float leak or mis-adjustment
3. Pump Pressure

That's really all there is, but since lots of carbs run 7.5 lbs I would highly suspect one or even both of the first two.
I have that exact same carb on the Wife's 72 Dart and it runs 7.5 lbs without any problem.