318 Retards Under Load?

OK, I am going to take this diagnosis a new direction. Try manually shifting into 1st (L1) at a dead stop and launch again. If it is the same feeling then it is not the transmission. Check and make sure the new ignition box has the cover properly grounded. If it is a painted box going on a painted inner fender, I doubt it is properly grounded. Did the car originally had electronic ignition from the factory or was it converted? Get a probe and measure the actual voltage going to the electonic box. If you have 13+ volts you should be fine. If you are finding more like 10-11, someone spliced in the power after the ballast resistor.