It's very easy to do by hand. And, if doing entirely by hand there's no need for planting in blocks, unless it's to conserve garden space. I plant in single rows, which makes hand pollinating very easy.
Some years the plants don't tassel and silk at the same time (I've had one variety produce silks one month after tasseling
). So, I freeze pollen until it's time to pollinate.
Also, if attempting to save OP corn, I think 100-200 plants are needed to keep a variety stable.