Look at the end of the day, if we do go in as named, I think we're probably OK.
The reasoning for playing Casboult + big 3 is likely because Cas killed it last week, the big 3 are relatively mobile for their size and so we can leave McKay full time.
Unfortunately, I think it means we are too tall and slow. Fast/agile for your size doesn't mean you're actually fast or agile.
Unpopular opinion, but I think if we are determined to keep McKay as a pure forward, we end up choosing best 22 between McGovern and Cas.