If the system was open for more than 12 hours or so, I would replace the accumulator, or vacuum it overnight. 30 minutes won't pull all the moisture out that accumulated.
This will be a bit long, but I hope it will help you understand what is happening. Or would happen if you had the correct amount of refrigerant. Ideally, the high-pressure refrigerant condenses to a liquid in the condenser, and pools at the bottom. Liquid refrigerant has a much higher thermal density than the gaseous refrigerant coming out of the compressor and in the top half of the condenser. So most of the heat extracted from the condenser actually comes from the liquid refrigerant, not the gaseous refrigerant.
Since you didn't fill enough, there isn't much (if any) liquid R134a in the condenser, so very little heat is removed from the refrigerant. This results in the liquid being much hotter than it should be, in turn resulting in the high low-side pressure after the orifice tube. The refrigerant is still hot. This is why you get 70° air out the vents.
The other problem I see is you listed 1.6lb as the capacity, but my service manual shows 1.8lb for a 2001. And you only have 1.1lb in the system, so about half what you need. Put another 10-14 oz in the system (up to 2.0lb works well in these trucks).
At 90°F outside, a low-side pressure of 40-60 PSI is considered normal, and a high side pressure below 300 PSI is also normal. I bet it stays below 300 PSI as long as you have good airflow through your radiator. The ECU will disable the compressor if the high-side pressure goes too high, I think around 400 PSI.
As for the compressor, most come with an "average" amount of oil, and you shouldn't have hurt it. If you replace the accumulator, that can hold a lot of oil, and you should measure how much comes out, and put that much new oil in the new accumulator. Ideally you do the same when you replace the compressor. Since you've run the new compressor, oil will have migrated around the system, and you don't really know how much you have anymore. You can drain all the oil from the system then flush it, and start with the GM spec, or realistically, just add an oz or 2 (max) and you will probably be just fine.
If your previous compressor failed, I strongly recommend replacing the condenser, accumulator, orifice, and liquid line as well as the parts you already installed. The debris will pile up in the condenser, liquid line, and orifice tube and result in high high-side pressure and very low low-side pressure. Since your low-side pressure seems OK, you might have been lucky. Did the compressor grenade, or have a clutch failure? If the compressor was working internally (like a simple clutch or bearing failure), then the system is still clean inside, and you can relax.
