I took apart a very original slant 6 AC car and it had no harmonic balancer bolt....The dust build up in the hole was an inch thick....it never had one from the factory....Should it have? Is a slant 6 bolt like a small block bolt only longer? Thanks

Most slants had no bolt, a few did. It uses the same bolt as the small block.

I have a small block bolt but it appears to be too short......

Very few slants came with a bolt.
I've used big block bolt and washer, not in fear of the damper coming off but to make it easier to turn the motor over by hand.

IIRC, the bolt you can use is a 3/4" fine thread 1-1/4" long.

I was wondering this same thing...when I took the 225 out of my Duster, there was no bolt..I was ...then on a recent trip to the local yard, I looked at quite a few slants, & none of 'em had a bolt.