The 6 cyl and 8 cyl compressors have the same mountings and are physically interchangeable for later model wags having the sanden compressor.
The difference is number of pistons and displacement (ie- cooling capacity).
Compressor for 6 cyl applications (SD507) is five piston unit with 7 ci displacement. 8-cyl applications (SD709) is seven piston with 9 ci displacement.
That york in your 2nd link is NOT a direct fit replacement if your wag is a later model already having the sanden.