GPD makes a good product, they're all I installed. AC Delco R4's are rebadged four seasons units. The scroll compressor is nice for a single air unit, but it's not big enough for a dual air. Replace the condenser with a modern parallel flow design, I would replace the evap for the front as it is most likely half plugged or worse, a severe duty fan clutch as dual air units have alot of heat to pull out of the condenser, put a 99 expansion valve in the rear AC to prevent slugging and/or the rear lines filling up with oil, and put in a 99 .062 orifice tube in the front AC to improve idle AC performance.
I would run a new GPD compressor, or if you want to get fancy with it, Dirty Dingo sells brackets to install a 508 Sanden compressor(not sure if there is enough room for the 709), but it will not have as good of performance at idle as the Sanden compressors are not real efficient at lower RPM's, and the 508 is only available in up to a 9 cfm displacement.