Does it back fire at all? If you know there is rust in the tank you will have to pull the tank flush, clean and seal it, plain and simple. Your highly likely running lean and the car simply can't be supplied enough gas under load.
If u don't clean the tank, you will have the same problem with the new motor. Likely the injectors are gunked and nut rusted clogged. Unless you don't have a filter on the supply line to the injectors.
Additionally you will need to check the screen on the fuel pump.