I assumed this was a common modification to Subaru sandcars. Surprised no one has the answer. I built a Subaru mid engine and my own alternator bracket, but I don't run power steering. I used a piece of string to measure belt length. I got it wrong the first try (of course) but the parts store was happy to exchange for a different length.
(Having problems with posts sending, I apologize if this comes through multiple times)