Probably 5 out of 10 years they won't even get this big... especially if good soil seed contact is not achieved.
Do you have access to a drill? How about a vertical tillage tool?
If you don't already have experience with cover crops, I recommend that you start with a low risk cover cropping strategy such as drilling cereal rye into corn stalks ahead of beans.... or spinning/blowing on rye (perhaps mixed with DAP) and then lightly incorporating with a vertical tillage tool.
If you really want to add a brassica to the rye, start with something cheap and relatively winter hardy like rapeseed.
After you have a few years of success with CC, if you still want to try planting CC while harvesting... go for it!