Hi Mike,
Maybe not directly helpful but here's the setup on my 521:
and
It was done using a combination of 460 F250 pulleys and 5.0 pulleys.
It has power steering so I opted for a two-belt setup.
A single belt setup using the alternator to tension the belt would be a nice setup.
You may want to take a look at the 5.0 pulleys and see if they can be adapted to your needs. Probably cost a lot less plus steel pulleys are more durable! They're powder coated black and have logged over 8,000 miles with no problems.
Regards,
Tom