Just as slow as before my mods..

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.
Yes. Which is exactly what I think may be going on here. None of the kick down and throttle cable bracket is for a 4 barrel. The car a originally a 6 cyl converted…
‘attached is a pic of the OP’s car as he bought it.

IMG_4250.jpeg