No need to convert header size from 76 to 80 as miners support blob size 80.

src/chainparams.cpp

@@ -203,9 +203,9 @@ public:
         pchMessageStart[3] = 0xe4;
         nDefaultPort = 19335;
         nPruneAfterHeight = 1000;
-        genesis = CreateGenesisBlock(1550789769, 10030, 0x1f0fffff, 1, 500 * COIN);
+        genesis = CreateGenesisBlock(1550900037, 3033, 0x1f0fffff, 1, 500 * COIN);
         consensus.hashGenesisBlock = genesis.GetHash();
-        assert(consensus.hashGenesisBlock == uint256S("e1f4cbc6fa8f2cc528387496120ff3aea1e4cc1e4c31f8af619d5c30a8a326f4"));
+        assert(consensus.hashGenesisBlock == uint256S("584a067cf7ea6daf30c351a46c6b7403d6fdc0e6e953cf7531d11eb7d834546c"));
         assert(genesis.hashMerkleRoot == uint256S("3cf6c3b6da3f4058853ee70369ee43d473aca91ae8fc8f44a645beb21c392d80"));
 
         vFixedSeeds.clear();

src/primitives/block.cpp

@@ -23,26 +23,18 @@ uint256 CBlockHeader::GetHash() const
 uint256 CBlockHeader::GetPoWHash() const
 {
     uint256 thash;
-    // Convert it to Cryptonight header format.
-    uint8_t cnHeader[76] = {};
-    // Bitcoin header size is 80 bytes, while CN is 76 bytes.
-    // To reduce it to 76, combine nVersion(4 bytes) and hashPrevBlock(32 byte)
-    // into 32 byte SHA2 hash.
-    uint8_t combineHash[32] = {};
-    CSHA256().Write((uint8_t*)BEGIN(nVersion), 36).Finalize(combineHash);
-    memcpy(cnHeader, combineHash, sizeof(combineHash));
-    uint32_t offset = sizeof(combineHash);
-    // Copy 7 bytes of hashMerkleRoot
-    memcpy(cnHeader + offset, BEGIN(nVersion) + 36, 7);
-    offset += 7;
+    // Convert it to Cryptonight header format, i.e. nonce at 39th byte.
+    uint8_t cnHeader[80] = {};
+    // Copy nVersion, prevblockhas, and 3 bytes of merkleroot.
+    memcpy(cnHeader, BEGIN(nVersion), 39);
+    uint32_t offset = 39;
     // Copy nonce.
-    assert(offset == 39);
     memcpy(cnHeader + offset, BEGIN(nVersion) + 76, 4);
     offset += 4;
-    // Copy the rest of hashMerkleRoot (25 bytes)
-    memcpy(cnHeader + offset, BEGIN(nVersion) + 36 + 7, 25);
-    offset += 25;
-    assert(offset == 68);
+    // Copy remaining 29 bytes of hashMerkleRoot
+    memcpy(cnHeader + offset, BEGIN(nVersion) + 39, 29);
+    offset += 29;
+    assert(offset == 72);
     // Copy the rest of the header (timestamp and bits).
     memcpy(cnHeader + offset, BEGIN(nVersion) + 68, 8);
     // Compute the CN hash.