I have been talking to a couple of the AC places (Vintage Air and Classic Auto Air)about a conversion for my 79 Power Wagon and I have gotten 2 different storys - 1 is that the sanden has a smaller displacement than the stock unit and it will not work effectively (so essentiall I have to go with a full aftermarket setup)and 2 the sanden will work just fine. Thoughts or comments??

classic told me the stock stuff inside should work fine and if I wanted, all I needed was the engine side.
I got the whole thing because of other reasons, mostly because I changed cars from when I talked to them the first time, but besides that I didn't want to bother with the vacuum lines and cables.
Also they recommended I keep my original condenser when I had one since it was more efficent than the newer ones.

I have a sanden compressor on a 71 Challenger R/T with the 71 evap and condenser. It works very well.

I also have a 67 Barracuda Convertible with the complete Classic setup.
The only difference is that the fan is a little better on the Classic unit.

I'm running a Sanden on a '70 Challenger with stock AC in the passenger compartment. Works great.