I'm in the process of disassembling a 345 that I took out of a 71 to put in to my 68 Stepside. My question is how do you know when you need to replace the pistons? The ones I'm taking out of the 345 are rusty and the port they are in are rusting around the ends. I'm pretty certain they they will have to be bore out which makes me think I'll have to buy new pistons. Thanks for any help