One positive thing to keep in mind is that if these are going to be halved/grilled, they don't have to be huge birds. My customers that buy them for grilling usually want them around 3 pounds dressed. If you push them a little, they could easily reach that size in 6 weeks. I also think I would go with all cockerals instead of str8 run to shorten the growing time. If you have only done a batch(es) of 50, 300 is a big jump. You will be surprised how many more feeders/waterers you will need to keep them eating/drinking.
This may be difficult, but it's very doable.
This may be difficult, but it's very doable.