Harmonic Balancer Help Please!

So Ive looked through all the previous posts about this and have failed to find an answer. I have a threaded crank on my 74 225. I rented a harmonic balancer from the local parts dealer and it had a 5/8 threaded bolt that was too small, so i tried the 3/4 and it was too big or the wrong thread. Ive tried a small block bolt out of my 318 and it was to big, it threaded in about a quarter turn then stopped. I pulled the bolt out and the first few threads were messed up as if it wasnt the right pitch. If you could tell me the exact bolt size and thread pitch it would be greatly appreciated! Also I did clean the threads and it still didnt thread in.