'70 318 BBD falls flat under load

I had the same problem with my daughter's 1974 Dart Custom 318 as what you describe. I ended up adjusting the step up piston little by little until the carb performed under all conditions. I wrote down my starting position of the adjusting screw and then noted which way I turned it and how much and what the result was.
It took me a lot of fiddling around with it (it was an off the shelf reconditioned carb) until I was satisfied.

A little while later, my daughter ran the car into a utility pole and bent the floors and A-pillar so bad I had to get rid of the car. But the carb was adjusted! LOL

Get an Edlebrock intake and put a 4 barrel on it.