Glad to hear you got your engine/vibration issues all worked out and that new engine is purring nicely.
Not totally sure this will help with your PS dilemma but maybe some worth a try.
I use GM fluid # 19329449 on my Delphi 600 box with no problems but I use a Saginaw pump.
Found this online:
https://itstillruns.com/causes-power...w-6038804.html
Maybe air in the PS system and needs to be bled?
Pressure on your pump needs to be reduced?
https://www.youtube.com/watch?v=CQuNkSDSNCs
https://www.summitracing.com/int/parts/brg-899001
Re-routing PS lines?