Use a reamer as mentioned. A 6” chucking reamer will barely break thru, but it will work fine. Just chuck it at the end of the chuck mostly. Also, use an air drill. It gives you better control, and is less torquey than electric or even a cordless. You’ll also appreciate the smaller drill, when you team #5.You can also help by just chamfering the edge of the hole, either with a file or bit to help reamer engagement. Ream with steady pressure. Don’t horse it, but keep it steady, and keep pulling the reamer back to clean the flutes. You don’t need lube, the cast iron doesn’t need it. When done, put a 1/2” cartridge roll on an extended shank, and go in and touch all the inside of the galleys. Reaming WILL leave little shards of metal, and you must clean them off.