First thought is that was too much R-134a - generally you want about 70% as much as R-12.
Second thought is that a compressor was changed out with you supplying the compressor for $475 (cdn ?) and it was neither flushed nor oriface (a $2 item) tube and dryer replaced ? That just is not right. I replace the dryer any time a system is evacuated. Of course living in Florida and Texas makes a/c a necessity.
Some of the kits claim to be able to just changeout now without flush but *any* time there has been a known compessor failure you should flush.
For some inteesting pictures of oriface tubes see:
That said if the hot-cold servo binds (rod to diverter is on top of a/c programmer behind glovebox with ATC) neither the a/c nor the heat will work properly yet is just hard enough to get to that is rarely checked. If the pressures look right and or on MAX HEat and not getting much, check the diverter.
If you push "AUTO" and nothing happens the first thing to check is the a/c clutch. If it is not engaging then the next stop for me is the a/c compressor relay followed by the pressure switches - you really need a schematic.
Hopefully the 90 Reatta/Riviera Service Manual will be on line soon (waiting approval from GM) - the HVAC section and most of the drivetrain is the same as for a Bonneville.