I'm not completely sure, but I had an SSD (WD Black SN850P with Heatsink for PS5) and it didn't fit, I had to remove the heatsink. So I would assume that the 990 Pro with heatsink won't fit either.

Exactly, only SSD and RAM. And you also need a power cable, I think it's called a "c5 power cable", it's a standard one that you often have lying around.