This issue was covered in detail in a previous thread. Some re-builders are using small block Ford (289/302) distributor cores as substitutes for Y-Block distributors. I have seen one so far where the shaft was repinned. The difference is in the distance from the mounting face of the distributor and the end of the drive gear. This causes two potential issues. One is with the distributor gear and cam gear alignment. The other is the short shaft does not allow the distributor gear to bottom out and ride on the block as it's suppose to. These Frankenstein distributors may work for a time, then fail.
In my opinion, your best bet is to find a known 1957 and up Y-Block distributor and rebuild it yourself. They are around, but are getting pricey. I see them on EBay for $150-200. Brand new distributors are available for big bucks. Here is a photo I took of a real 1957 Ford Y-Block distributor and an incorrect small block rebuild sold as a Y-Block replacement. As you can see, the shaft on the left distributor is 1/4" too short. I purchased the incorrect distributor from Macs a few years ago.
