That's right. The main reason you want to support the control arm in the rear is that the shock drops out through the spring and you want to get as much clearance as possible between the bottom of the spring and and the ground in order to remove the shock. With the shock at full extension and the suspension hanging as low as possible it will be very difficult to get the shock out without digging a hole under the car. Since the shocks are old, compressing the shock to shorten it and get it out should not be a difficult issue. Putting the new shock back in is another issue. The shock needs to be compressed to fit back up in there. If your shock does not come compressed, compress it, and using light wire, hold it in the compressed position. Once the shock is inside the spring, reach in with a pair of cutters and cut the wire, allowing the shock to extend. But be aware that once you do this, that new extended shock will be very difficult to remove so make sure everything is correct and ready to go before cutting that wire.
If you do this on a lift, all these issues are irrelevant because you'd have plenty of space to drop the shock straight down.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13