Actually, this problem isn't limited to Model A blocks. Early Model B blocks have the same problem; it was corrected starting with s/n 173828, which IIRC corresponds with about midway through the 1985 model year.
BTW, I'm getting my data from a Ford publication that was issued in '85 or '86. It was posted in .pdf form on TDS a few years back. If anyone's interested, I can E-mail it or upload to the Web and provide a link (although I'm not sure when I'll get a chance to get to it). I've never taken the time to read through all of it, but there's some VERY good info in the parts that I have read. I took a screen shot of the breakdown of the various improvements made (as of the date of publication) along with the corresponding serial numbers. The "1984 Exhaust Emissions Engine" mentioned in the table is the Model B. Actually, I think you can put Model A heads and pistons on a Model B block if you wanted to; to my understanding, the differences are in the heads, the pistons, and the precups.