I think your problem is overcharged. I have a 84 500SEL which I had many problems trying to get it to cool.
I tried everything (like you) and nothing seemed to work. Now let me be truthful and say I was using R134. I then tried Freeze 12 one time and things were worse. Flushed everything again, replaced the dryer, then put in 1.5 lbs of r134. I set the RPM to 1500. Checked center vent temp after about 2 or 3 min. I forget what it was, but it didn't matter (as you will understand in a moment.). I then added 2 ozs and again watched the center temp. It droped 2 or 3 degrees. Again added 2 ozs. Again temp droped. I kept doing this until the 2 ozs didn't drop the temp anymore. I'm sorry that I can't remember exactly how much I put in the system, but It doesn't matter. Now the A/C system works great with the R134. I will agree it is not quite as cold at idle as it may be with R12 but but when moving it works just as good.
Going about 50 or 60 mph the center duct temp is about 42 degrees and at Idle it goes up to about 48 or so.
If you try this method, you will need a good scale to mesure the amount of freeon put in each time and watch the guages very closely to make sure pressures dont go to high.
I have used this method on several other conversions and all are working great.

Now I am sure the same method will work with R12.

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84 500 SEL (307,xxx miles)