To answer the first question, yes, you can use JB Weld to bed your rifle. The acraglas kit has more than enough resin and hardener to bed several rifles, but if you really feel a need to do it your way, go ahead and use the JB Weld. I have seen rifles successfully bedded using dozens of varieties of epoxy, fiberglass resin, acrylic resin, and even hot melt glue.
To answer a question that was not asked, the JB Weld will probably not stick to the SPS stock. You would be better off with a different stock.