Progressive has an extensive on-line catalog. It probably will not list a shock for your bike as it didn't for my 85ZN700. I put the bike on the center stand and measured the length and eye bolt diameter and looked for one that would fit. There were 3 different spring selections depending on the load. I suspect you just need shocks with stiffer springs. You can probably find what you need there.
1985 Kawasaki ZN700 Shaft, 4cyl
1989 Yamaha Venture Royale
2007 Yamaha Royal Star Venture