I had a '74 SSX 340 for a while. They are hefty, narrow sleds and also top heavy because of the tunnel-mounted engine. Not a combo that makes for good handling. It all adds up to a really tippy sled.
The rear suspension on my sled worked fine. The springs were new, and the shock was in excellent shape. It worked really well for such an early design. It was comfortable at reasonable speeds.
I've run ski spreaders before. Not sure, but I THINK they came stock on my '79 El Tigre 5000. They were made of steel plate and widened the stance by 2" per side. These sleds are quite a bit faster and more powerful than the SSX 340's, with similar weight. Even so, the ones on my sled NEVER bothered, even on rough trails.
I also added a pair of 2" aftermarket (steel) ski spreaders on my Scorpion Li'l Whip, which was a flyweight of a sled but had similar rear suspension to your Chaparral. It handled a LOT better and the ride wasn't hurt at all.
IMO, the ski spreads probably won't fail, not even aluminum ones. It might be a good idea to keep an eye on your ski spindles for signs of bending.
Hope this is some use to you.
-Strato-