Grasshopper how much is your song?
I need one on that Aprilia.
You'd be fine I'm guessing on that shock. As long as you look at a few things.
As I understand it. When you order a Penske shock, you give them your weight, bike and such. They then build YOUR shock with the proper valving and spring.
Penske shocks are rebuildable and can be adapted to other applications. If you had the shock and wanted to do this (as I'm considering on the Aprilia) it might be worth it. If you have to buy one second hand, have it refreshed, respring it and then buy the parts to adapt it. I doubt the cost justification is there.
Now here is what you need to look at Jason. IF this shock came from say a 600 RR which has a large leverage ratio, it is valved for heavy springs. Soooo. Me on a 600 RR might need an 800 lb spring. If you planned to put this on a GSXR where my weight of rider would need, say a 525 lb spring, the valving will probably be WAY off.
So... If you just need to go within a few spring rates, I'd say it's probably fine. If you have to do a large change, the valving will probably be off.
Also make sure the mount points are the right type. This may not be an issue. But on my FZR, it uses a U shaped lower clevis, other bikes have the center mount with the ball pivot, like my SV has at the top. Some bikes have a center bolt at the the top mount. This bolted to the frame. It also made it easy to shim for more ride height. But my point is, make sure it will fit your application. I'd hate for you to buy something and it won't go on the bike.
Yes, if you were running on an AMA grid, you'd probably change valving with big spring changes. I blew up my shock last year. I borrowed the Penske off Wink's SV. His was just refreshed. I mounted it up with my spring. I ran about 1.1 seconds slower. I couldn't get the bike to finish a corner. His Penske is sprung for a MUCH lower weight then mine. My guess is that I was blowing through the valving and I was losing to much ride height.
Remember, the main point of the springs is to hold the weight of the bike up. The valving and oil levels / weight do the rest. That's what you will read. But once the valving is in the ball park, the most likely change you'd see other then turning the knobs, is likely a spring rate change.
It can be done, but if your doing this to save money, be sure to cover all your angles.