After reviewing a plethora of GMC-Net messages and a number of web sites I decided that the real problem is not cooling capacity but lack of airflow on the evaporator side of the system. My solution was to add a couple of additional air ducts to the passenger compartment, one on the drivers side (in the floor just above and to the left of the brake peddle) and the other on the passenger side (just below the glove box).
Someone had suggested using industrial vacuum cleaner blast gates and 3" hose to install fresh air vents. I thought that would also work well for my air conditioner upgrade. I ordered the hose (DF100), 2 blast gates (DF121) and 4 clamps (DF117) from Trendlines (woodworking tool supply company, 800-767-9999).
The blast gates are a 2 piece aluminum affair with a moveable steel plate that acts as a cutoff valve. I unscrewed the two halves of each blast gate and discarded the steel plates. I cut off the excess aluminum so that all I had left was 4 square flanges. Each flange has four mounting holes and a 3" hose port.
I removed the coolant tank and the washer fluid tank from the front of the HVAC box. I remounted the coolant tank to the fire wall on the far right (above the engine battery). I am planning on replacing the washer tank with a smaller one and mount it to the far left fire wall. I may replace it with a 12v solenoid valve connected to the house water system. I rarely use the windshield washer so I have not decided yet.
I used a 2 7/8" hole saw with an extension to make 2 holes in the front of the HVAC box and two holes in the floor. I used the flanges (made from the blast gates) as templates to drill mounting holes for each flange. After attaching all four flanges I cut, routed, and clamped two pieces of 3" hose to connect the flanges on the HVAC box to the flanges in the floor.
I needed a way to control the quantity and direction of airflow so I looked at various grills and vents. I decided to use a plastic "eyeball" style air vent typically used in aircraft. I found one (part number 13-03800) sold by Aircraft Spruce and Specialty (800-831-2949). This vent will allow you to adjust the airflow and direction for each air outlet. I mounted the air vents with Velcro so they can easily be removed if I need full airflow (in the event of a roof air failure).
I also moved the power source for the HVAC blower's high speed circuit breaker from the battery circuit to the alternator circuit (center post of the battery isolator). This reduces the load on the battery isolator and gives the blower motor an extra 1.5 volts or so. Some of the increased voltage is from bypassing the diode drop in the battery isolator and some is from bypassing voltage drops in the wiring harness.
This upgrade directs cool air from the cool side of the evaporator coil directly to the passenger compartment. IT WORKS EXCEPTIONALLY WELL I don't need to run the roof air until the outside temps reach the upper 80's. I don't need to run the dash air blower on high until the outside temp gets above the mid 80's. Before this upgrade I was running the roof air anytime the outside temps got above the lower 70's.