progressive shocks

[sam78gs750]

So I'm looking into shocks for my bike because my suspension is TIRED... the Progressive website lists part #12-1204B for rear shocks for my bike. so stupid question: do I have to use that specific part # or can I just use any 13" shock from their 412 series? reason I'm asking is I found a few 13" ones on ebay, but most of them are listed as taken off of harleys or some other bike and have different part #'s.
Also, if anyone can offer some less expensive alternatives I would appreciate it. In searches I keep seeing the name MDI pop up, but all the threads say that they're pretty stiff.... I'm about 150 lbs. and have back problems, so I'd like to avoid those if possible.
Edit: Ok, so I looked a little closer at the pics of the ones on ebay and realized that the mounts are different than the ones on my bike...

[crapwacker]

You have to look at the length, and the mounts. Is the springs are too stiff or weak, that can be changed. But want the length to stay the same. Z1 enterprises has IKON shocks that are $275