A few months ago I acquired a clean 2 door 1990 montero that sat for 5 years with the block and heads rebuilt but not together. Anyway yesterday I was out putting miles on it and all of the sudden it started loosing power... I heard some squealing and it died. It will start with oil pressure but it sounds bad....rattles and some squeaking.. There was no warning. Run really quiet. I read that the oil slinger plate should go to outside of timing belt cover and I may have put it behind cover........ Will that cause what I described? What steps should I take to pinpoint my problem.

By oil slinger, do you mean the stamped thin metal flange? If so, it goes in front of TB crank sprocket, after the timing cover is installed, before the serpentine pulley goes on.

I don't think it's your problem. I'd remove the serp belt and try turning each of the accessory pulleys, as I think your problem is one those (PS pump, alt, water pump, a/c compressor) is most likely your problem.

I just now removed alt belt... AC not on yet and power steering belt loose.... No help. It sounds like a small block Chevy with un-primed hydraulic lifters and rattling push rods. It will kinda run if I work the gas pedal.... I was surprised cause it was running so quiet and smooth before it stranded me in BFE with a $185 tow bill.

Is it possible to loose oil pressure to one head?