I've heard it said that 350 pistons are hard to come by. I've never looked. If they are readily available my advice would be to buy forged pistons. I've had issue with cast and will never use them in another motor I put together. Also, you have to know the exact dimensions of what you're going to put together before you purchase as far as compression ratios and such. There's a lot to be considered. Pin height, ring pack, valve relief, how far in the hole they will sit, deck height, chamber size of the heads you will be using, bore and stroke. Even your cam specs as far as opening and closing events. It's the difference of how much of your charge is bled off early or late. It all adds up.