8 3/4 Sure Grip ratio change

The part throttle upshift is controlled primarily by the governor pressure, fighting the line pressure and delayed by the KD adjuster.
Reset your KD to the max. If the upshift is still too early, you will have to get inside the tranny.
If it busts the tires loose on the shift, delay your secondary opening, or do something about your rear suspension.533ftlbs @4000 is a heck of a problem for street tires,lol

Now, IMO, trying to solve an oil leak at the rear mainseal, with crankcase evacuation is not a very good idea. Your system will be pulling in unfiltered air and circulating it all through the engine. And the air is coming from just about the dirtiest place imaginable, right next to the pavement.