I think you will have better luck with a food plot if you take the time and do the work needed to get it out in the field. I have seen a bunch of people try to get something going inside timber and very few of them have any luck with it at all. As far as the johnson grass goes, I would spray the area with roundup, wait until everything is dead and then turn the soil over (you may have to rake the dead grass out of the area). Once you turned the soil over wait another week or two and then respray with round up again to make sure all new grass growth is taken care of. At this point you may need to add lime (your soil sample results will let you know exactly what needs to be added). Turn you soil over again, spread fertilizer and your seed. I would suggest going with an annual seed, it costs a little more money but you get better/cleaner plots when they are reworked each year. Good luck and I hope you get on a tank!