Good luck with this one, sorry. I have not seen tension rods for a 320 or 520 (interchangeable, same part number) for sale recently. Okay, last 18 months or so. Best bet is to have a machine shop make these for you and this is the route I'm going. I just have not had the time to get out to the shop.
I also do not know if any tension rods from the 620 or 720 work, I want to say they do not. The 521 might work, but they might also be metric threads vis the SAE of the 320 and 520.
I'll get to the shop next week to get quote and time frame for fabrication and then get back with you.