Having 60% of the mining power means you mine 1.5 blocks on the "false" chain for every block on the "real" chain. You don't need to mine 6 blocks in a row aga

Having 60% of the mining power means you mine 1.5 blocks on the "false" chain for every block on the "real" chain. You don't need to mine 6 blocks in a row against the other miners, you just need to have a longer chain. So you simply fork the chain, and then submit that as the "real" chain. With 60% of the mining power and a 6 block deficit, you'd overtake the main chain in 120 minutes (approx; 12 blocks), since you're making up half a block of deficit per block they mine. If you really had 60% of the mining power, you could undo 3 days of transactions in about a week of mining.