Shredding belts.

Thanks for the good info update. Good belt brand.... as long as it is not a Dayco Top Cog type..... OH CRAP... it IS a Dayco Top Cog belt. THROW IT IN THE TRASH!!! NOW! NOW! NOW! GET A NAPA PREMIUM BELT. These Dayco Top Cog's are nothing but trouble under heavy belt loads. They will slip badly without making any noise. BTDT...twice. It may not solve everything, but that is about the worst belt made, in my experience.

FWIW:

• If it slips this under rapid acceleration of the engine RPM's, but not under slow acceleration when the car is sitting still, or at steady state RPM's low or high, then it is the rotational inertia inside the pump's innards that is make this slip. But see the next.....
  
• Have you carefully checked for slipping at high RPMs? If so, then the total pumping load is the issue. That could be the fluid.
• Peak loading on the pump takes place when turning. The faster the turn, the more load. So the faster steering rate of the new box is part of this.
  
• Same thing applies if you turn the steering wheel with the car not rolling: the load on the pump goes way up. Since you mentioned your son, I'd be suspicious that he often turns the wheel with the car not rolling.

Bottom line, it now sounds like you are just exceeding load rating on the belt with a crappy belt; they do have power limits. The belt has to transmit power to 2 loads here; the smaller pulley is the one that will normally slip first. The added fan load seems like it ought not be a direct issue as these fixed fans were used for a long time in these cars, but it would add to the belt heating which might exacerbate problems at the PS pulley.