Unlike Bitcoin, the block header that is hashed for PoW in XMR doesn't seem to be the same as the block header structure in the code. In other words, this header structure seems exclusively used for PoW. Its format is as follows:
|major_version||Block major version number||Variable|
|minor_version||Block minor version number||Variable|
|prev_id||Hash of previous block (NOT the PoW hash)||32|
|nonce||32-bit number used when mining||4|
|txcount||Number of transactions included in the block||Variable|
The variable sized integers are stored in Monero's varint format.