You'll be riding pretty low, but it could be done.
I hauled a bunch of dog food up to a landowner one time as a "thank you" for letting me access some lakes through their property. I had ten 50lb bags of dog food, and another 100lbs of bird seed, plus our backpacking gear @ about 80lbs in the back of my '97. That was when it was sitting on 2.5" spacers and '99 tall coils. So, a total of 680lbs (and well over 800lbs if you include my hiking buddy's weight as a passenger) and it was riding just off the bumpstops the whole way up.
That setup sat about an inch lower in back than the SS 1.2, and the spring rate on the '99 talls is about the same as the LC coils that come with the 1.2.
Just drive very cautiously, especially over any big bumps. It'll handle strangely with all that weight in the back, because not only does the rear sag, but you'll notice the front will actually see a bit of "lift" and that will put your alignment off for the duration of the drive.