Yes, afraid so.
40mm wide shoes fit in to a 203mm diameter drum.
50mm wide shoes fit in to a 200mm diameter drum.
In other words, the 203x40mm shoes won't fit in to a 200x50mm drum, because they're a larger diameter. It doesn't seem a lot, but trust me, you'll be cursing.
My suggestion is to order a drum that matches the majority of the parts you already have (I'm guessing the 200x50 option, as you've ordered the brakes for that?). An OE blank drum shouldn't be anywhere near that money tbh.