Lower gears and still slugish.

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.