How bad is your block? Generally speaking 351 pistons are harder to come by than 305 pistons. Anyway, you are talking about going from 4.25" bore to 4.56", that's an awful lot of material to remove and because of the high nickel content of the V-6 block they don't cut easy. And that's assuming there is enough cylinder wall to go that far as others have pointed out. A quick check on Ebay shows 2 .060" 305 pistons available now. Your best bet is probably just keep looking for 305 pistone, they do show up.
https://www.ebay.com/itm/31265662593...hoCn-IQAvD_BwE
I have no connection to the seller.