new balancer too tight

Before going any further measure the I.D. of both damperers and compare. Are you using a new woodruff key, if so put it in the groove of the crank nose and measure how proud it is then compare that to the depth of the keyway on the dampener. If the dampener keyway is not cut deep enough the dampener will hang up and not go on. Also, you could measure the size of the old key and compare the keyway depth on the new dampener to the old one.
Yes, as Green above said get a new fine thread bolt 1/2" longer and draw it up with that until you can use the stock short bolt and get 5-6 threads on it. Also use the hardened steel washer, file off any high spots, so you can draw the dampener on straight. I changed my dampener out earlier this year with no problem.

Terry