Have a look at the Firm Feel site. It will show lengths and types. I'm pretty sure there are lots of later cars that will interchange. I'm believe the late B/E stuff will work but not sure on the early B's.

You have lots of choices. If your current springs aren't broken/cracked, low budget but time consuming would be to buy a used set, disassemble and remove the main spring. Cut the ends off the donor main spring and simply add another full leaf to your current spring pack. A spring shop or several vendors will have the spring bolts and clamps you need.

Alternatively, take your current springs to a spring shop and have them add a leaf. This service is about $150 in my area.

If you want to start new, the MP extra heavy duty spring uses a bias design. I started with a set of the MP springs from Firm Feel. These springs are by design flat and spring rate was a little low for my application so I got a leaf added by the spring shop. I like the result.

Finally, Firm Feel, "Springs and Things" and other vendors can select or make a spring for your application.