Bummed out

You can't use the 6.1 pistons because there to big, the 6.1 crank is a great choice because its forged and has the same 3.580" stroke as the 5.7. For pistons you have a few options like probe Mahle weisco cp bullet series. I think one of those company's has a factory size piston, but you should have the cylinder measured to make sure there is no out of round or tapped before spending any money. You may need to have it torque plate honed to the next size. The rotating assembly needs to be ballanced and make sure to have the main journals measured and machined if needed. The stock cranks can be pretty good as long as there is no boost or nitrous being used, the stock rods and crank are junk. If your going to tear it down for new pistons you would be better to find a good used 6.1 crank and stock length rods you will have a forged engine that will withstand some nitrouse if you so choose to use it when your ready to go a little faster.