Shredding belts.

For anyone new 71 Valiant; replaced Slant 6 with a 360 out of a Lil Red Express. Alright, I'm not sure if anyone is still on this thread but I did some work last light. Took off the PS pump and checked pressure valve. The shafts\pulleys are different for my old slant one but I did change the relief valve because it had two washers under the nut rather than one. Not sure how to get that nut off without destroying the spool but that is another thread. I put everything back together and put a new Dayco 15455 Belt. A little longer than the 15445 but plenty of room to adjust. So I am 100% sure now that when my 18 year old son is running this thing (always) and wasting my money on tires, the belt is slipping on the PS Pulley. I can hear it exactly there when I rev the engine with the hood open. This happens under rapid acceleration not under real slow acceleration. It also matches where the belt dust has been forming. So...tracing things back, I build the engine (mild) about two months ago. Put Hedman headers on it that were really close to the PS gear box. I had some overheating problems with the old slant so I went with a new 3 core radiator and an electric fan. Fan must have been junk because it did not cool anything down for the slant of for my new 360. The headers were so close and things were so hot under there that the PS Fluid was boiling out of the pump. Up til now, no problems with current belt issue. So I put Synthetic PS Fluid in and it seemed to solve the boiling problem and the same time I installed a mechanical 7 blade fan, now that moves some air. NO overheating problems anymore. This is when the belt issue began. The rotation of the belt suggests that the crank pulls the PS Pump and the PS Pump pulls the Fan so I do not think the fan can be an issue with the PS pulley slipping, its too late in the system. That was a long description to get to my question. Can Synthetic PS Fluid in a Saginaw Pump with a new Borgsen PS Box add drag to the system? Basically could it be that the fluid is wrong? Oh. I did check the pulley before I put it back on, The V-belt matches the groove very well.