In order to adjust length on a Penske, the rear linkage rocker must be disconnected from the frame, shock and link; then the locknut loosened and the bottom "eye" threaded in or out while the shaft nut is held with a one inch wrench. I use a 1" service wrench and 1" crowfoot wrench. Consequently, once everything is back together, its impossible for the length to change. I have to recheck, maybe I was too tired last adjustment.

BTW, the rear of the bike must be suspended and its easier to align the linkage bolts if the rear wheel/tire is first removed (the swingarm can then be held in place with a heavy gauge wire hooked to the subframe and the axle through the swingarm). See pics of the gantry I made to suspend the rear of the bike. These pics are over 3 years old from when I first installed the Penske 8981.

 

Well it's a good question on how the shock could change length.
There is another way to take off the rear shock.... I take a peice of a 2x4 that is around 5-6" long (it's been a while so I don't have the exact length at this time) loosen the nut on the top shock mount and put 2x4 between the sub frame and the swingarm. Tap it in place and put the bike on a swing arm stand and you're good to go.

 

Whereas your method may work, it is not the prescribed procedure. And if the 2X4 should loosen (even if it is duct taped somehow), I shudder to think of the consequences. Secondly, your method does not provide the leaway necessary to readily remove & reassemble the linkage, particularly with a remote resevoir type shock. Some people suspend the bike from rafters when structurally adequate (and not).

 

Well I guess the the last time I pulled the shock & linkage I was mr. magic as there is pleanty of room to do it (and I even left the exhaust system hooked up) and it's much safer than hanging the bike from weak rafters or tring to work on the bike while it's swinging around. Why do you need more room for a remote resevior shock? The ones I've used have the same size shock body and spring as a standard shock, just an extra hose to run and a resevior to mount. I guess I missed something.
As for the 2x4 commong out, yeah it could happen but it would have to move over an 1 1/2" for that to happen, so besides the fact it has never moved while I've worked on mine, you would have to really not be paying attention for it to happen. I mean one of your straps "could" break also but it really isn't likely to happen. Besides if you ever tried it you would find out how stable it really is.
I guess my real point I was tring to make is for the guys who don't have a lift and fancy gantry or rafters to work from can still do the job with no real problems. So call it iffy if you like but I prefer to do it this way if for no other reason than I don't have a bunch of straps and hardware in the way while I'm working on the bike. YMMV