Lower gears and still slugish.

KickDown said

One you have verified the engine is in a good state of tune. Compression, ignition timing, carb, no vacuum leaks etc, I would begin looking at the torque converter. If the stator sprag is not holding, there is no torque multiplication going on in the converter & the car will be very sluggish on acceleraton. With your set up & the 3.73 gears you should be painting black lines all day long.

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With the gear change, you will need a governor change too.
What is your neutral idle speed, and what does it drop to when you put it into gear?
What are the specs on that cam?
I would also jack the wheels off the ground and check for dragging brakes; all corners.
I would also check the line pressure in reverse. At 2000rpm it should max out at 250psi or near it. This will tell you if the pump is any good. I didn't see that anybody asked about a stall test, so I guess that would be good to know as well,.