I think you are about to spend a bunch of money and effort on something that is not only not worth it, but going away from a performance driving experience.
The transmission you have is the closest ratio trans they put in an A Body. It’s a great box with an excellent gear spread. I’d change out the rear gears before doing transmission surgery.
To be honest with you, it sounds like your tune up is **** or your motor is tired. My 273 with 3.23 gears will crawl at idle and fuckin ROAST the tires off the line….and it’s an Automatic car…87,000 original miles on the motor completely stock.