I ended up pulling the motor and replacing the rear main seal - this time with an offset seal (Fel-Pro BS118291). There was a small groove in the crank surface - didn't look for it the first time around. Sealed up the oil pan and things are staying dry, finally.
Put the truck back together and started on the A/C install. Hopefully this information is useful for someone putting this kit in a GMC. Vintage Air sold me the standard Chevrolet C10 kit for a truck with deluxe controls - this kit won't work as-is with a GMC - helpful hints below.
The condenser installation is much easier with the grill removed. It's almost impossible to route the hard lines through the core support with the grill in place. I also had to bend a little flange on the back of the grille to clear the drier. The hard line from the drier to the condenser is easier to install with the condenser mounting bracket removed.
Mounting the compressor will be dependent on application...in my case it wasn't too much of a hassle. I am using the aftermarket 2.5" ram's horn manifolds and had to grind the manifold bracket just a little bit and play with spacers to get it in the right spot. All else worked as expected. I used some string to get an idea of belt length and ordered a range of belts to find one that was in the range of adjustment. Ended up with a Gates 7580 XL.
Plan out your sequence of hard line and bracket installation on the evaporator, there are connections that are really hard to access if you use the sequence described in the instructions. Once the hard lines are installed the rest of the evaporator installation is pretty straight forward. I purchased a Gates PN 18084 heater hose that has one end molded to fit a 3/4" fitting and the rest of it sized for a 5/8" fitting, since the VA heater core lines are both 5/8" diameter. I did use nutserts and button-head cap screws to hold the blockoff plates in place instead of the included sheetmetal screws.
Of course the included glove box liner isn't compatible. There isn't much room left behind there anyway. I'm planning to make some kind of new liner out of ABS plastic. I ran the wiring for the heater valve and the main harness down behind the unit and out through the firewall hole that was used for the original blower motor wire. I just drilled out a 1" plug to hold a wire grommet and fished the harness through.
The GMC controls are very close to the evaporator box. My truck came with deluxe controls so I wanted to keep them. The linear potentiometers that are used to convert from cable to electronic inputs hang down below the dash, and I didn't realize how visible they would be without the lower dash ductwork in place. Also, the bracket and mechanism take up a lot of space right where the defroster ducts want to be. I ended up running the driver's side defrost duct up and over the controls and the radio to avoid interference. Additionally, the linear potentiometers line up directly with the passenger side duct connection on the supplied under-dash louver housing. This wouldn't be a problem on the Chevy where the controls are closer to the driver.
I ended up ordering the 492090 trimline kit. This hid the linear potentiometers and allowed the duct to connect closer to the end of dash, eliminating the interference. I think an alternative would be to order the rotary controls kit and find a factory single knob dash panel. Then the dash would still look stock and I could probably use the rest of the parts that came in the kit, and the duct routing would be much easier. One more thing to note is that the driver's side vent can't be mounted all the way to the outside if you're using the stock parking brake lever. Not a huge deal unless you're really into symmetry.
I borrowed vacuum pump and a set of gauges and made sure the system didn't leak. It's charged now and blows cold - so far so good! Definitely a big enough system for a truck. I'm happy with the system but wish the VA crew would have suggested the potential conflict between the ductwork and the controls. Lesson learned, hope this helps somebody else!