Just as slow as before my mods..

4mulas said

Could you please post a picture of the carburetor from the drivers side, the part under the air cleaner showing the throttle cable and or any linkage etc.
this may be as simple as just not getting proper throttle actuation (full throttle)

I looked a pictures of the car as it was when you bought it and the throttle cable bracket and throttle cable didn’t look proper.

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First Dart I bought had exactly that problem. Was a 318 that the seller had put a set of J heads on, along with a factory cast iron intake and TQ, but had no throttle cable brackets, and no kick down linkage. The end of the throttle cable was cut off and wrapped around the arm of the carburetor. Putting the pedal to the floor gave it about 30% throttle, and it was in third gear by 25 mph. (904 automatic)
Once I got the proper cable, linkages and brackets, world of difference, even with the stock 7.25 differential, until it grenaded anyway.