So, did the Holley and HEI happen before or after the rear gear change? The gear change is a constant, the truck will perform the same day 1 after the gear change as it will years and miles there after. If your are saying, after a year and a half the torque drop in 4th gear is greater, then it's time to do a health check on the motor, tune the carb (not rebuild...yet), check the timing and the vacuum advance.
I doubt having two carbs on the motor will recover the performance it had with the 3.90's. But, in any case it should not lay flat in 4th gear.
