Are you looking for a rotary compressor? I believe the cast iron V-Twins were all the same size - only the brackets were different
Check out CVF Racing. They have the smaller Sanden compressor and the brackets / pulleys to go with it. Got mine there and the quality is first class.
Vintage Air has a bracket kit for the 69 and earlier smallblocks. From the pictures it looks like it moves the alternator over to the driver side. Just from the way it looks, you can leave the alt where it's at. The part number is 161004. I bought one for my 67 Dart 273, but my original compressor just keeps on ticking although noisy, so I haven't installed it yet. I saw one installed on a 69 Barracuda with a 318-it looked pretty good.