The under dash unit probably isn't aftermarket. That's the way they were done.
The Sanden 508 is a great compressor. I replaced my working York with a Sanden and I am very happy. Less weight, less drag on the engine, and no vibration. There is a company in Ft. Worth called Old Air Products. They are quite helpful and have all kinds of solutions. I had them rebuild my original drier.
I never found a cross-flow condenser that fit exactly. I did find a company that makes an aluminum condenser with twice the number of thinner tubes that is an exact fit. (Well, I had to enlarge one mounting hole.) Unfortunatly, I have lost that contact information, but I found them through the internet and they were very helpful on the phone. The volume of the replacement condenser should be very close to the original. That is important because you want your evaporator and condenser to have the same volume to keep the system pressures balanced properly.
The York to Sanden adapter you are talking about bolts onto the massive A/C bracket that mounts the idler pulley and York compressor. If you have this on the car, your alternator will be mounted high. When you bolt the adapter to the bracket, you'll need four spacers to make the compressor pulley line up with the dual belt crankshaft pulley. I used 1-inch spacers that I bought at Lowe's.
Bring your patience with you, it's tight under there. You'll want to put the condenser and radiator back in as an assembly, and last. You'll really want help for that.
Take a look at how other A/C cars are installed. Good luck.
Ray
'68 280SL 4-spd Coupe