Wow that's funny. I was going to ask this same question. I was told last weekend that a srx piston would work in a viper. I was told they are lighter. And a bit stronger then the viper piston. But didn't know if anyone has tried this or if there was any truth to it.
Best way to find outnis to look it up on a parts list. Partzilla.com is a good one for info. Look up the years and model and then the pistons. If they all have the same part number then they are the same. Some part number might superceed to another but should be close.