If you put 8 in and flushed everything out it sohould not bne that much of an overcharge. As a rule of thumb, the compressor will hold 4 ounces, the accumulator will hold about 2 the evaporator will hold about 2 and the condensor will hold about 2. Since I am not quite sure what compressor you have dont count on the 4 ounces. those specs are what I have found true for most of htese components. I hope you did not try to flush the compressor because that will mess it up good.
If you flushed everything clear and charged with 8oz of oil you should theortically have about 14oz in the system (cannnot flush oil out of desicant). I think spec is 10oz? (gotta look that up) so you have a slight oil overcharge here. This is enough to start messing up performance. Now, if you put a new copmpressor in the factory usually charges them with a full system charge of oil in the 8 to 10oz range. If thats the case, you added 8 to that and now there is way too much oil for sure.
BTW, what kind of compressor is in the truck now? If its still an axial compressor and not a radial one it might be a Sanden SD508? Or is it still a ford compressor (FS10/FX15)? if its either one of those you should be just fine as far as compressor displacament. If you changed it to a compressor without enough dissplacement you may have created your own problem here. Certain compressors made for gas engines (particualrly smaller cars) have smaller dissplacements becasue they are meant to be driven by a 4 cylinder turning 4k RPM on the highway... Something to consider.
Ken