I just bought a stock 66 chevy 327 engine a guy sold me so he could buy a rebuilt short block and my crank is threaded..?

You are correct SB Chevies pre 68 had no bolt in the end of the crank. First off carefully inspect the keyway slot inside the balancer and on the crank snout for any excessive wear, be sure to use a new key. Second be sure the balancer oil seal area is not worn with grooves---they make slick repair sleeves for this fyi. Then apply thin coat of silicone sealer around inside of balancer so to seal against crank good, apply engine oil to the lip seal and to the sealing surface of balancer. If you have an old balancer you can go face to face and hammer away or just use a small piece of 2by4 and get after it. Once the balancer seats all the way the lumber will bark back at you and have a different feel when you hit it. The danger of just using a sledge against the balancer is that you will likely swedge one of the bolt holes for the pulley. Finally put on the pulleys and straight edge them for alignment to make double sure balancer is bottomed out.