I completely agree with the above, do not add sugar blindly. As a future suggestion, I use jam bags and a large masher to make plum wine. That way the bulk of my fruit meat is easily removable but I still get the juice, and the pectic enzyme and, later, yeast can still eat away at the actual plums; I also leave everything in the primary for 1-2 days after removing the bags (in your case, that would be straining) to allow more lees to fall before transferring to a carboy and adding campden.
If you have a drill-mounted degassing rod, this may actually help liquify your must (in a bucket, use LOW settings on your drill). You can also add more water, with that much fruit it won't dilute it that much, but it sounds like you're going for a thicker wine. Are you keeping it in a warm place? Lower temperature will have a rapid thickening effect on plum mush.
Stirring will help, but you really need a watery solution to accurately read sg. I wouldn't add any more sugar (the batch of plum wine I just transferred to a secondary was 30#s with 4 gallons of water and I only used 7.5lbs of sugar; I may back sweeten a little, but even fermenting to dry it tastes pretty good and ended up with a pretty red-wine color and feel).
You COULD try just running blind; with that concentration I don't suggest it but it might work, though it will likely end up a pretty strong (or pretty sweet if using weaker yeast) wine if you added 8#s of sugar with that much fruit. Will you be pitching yeast or are you shooting for natural fermentation?