Where the hell do I find the serial number for my 351?

I've been looking for hours now, scraping off crud and dirt from under my truck and I can't seem to find it at all.

I didn however, get the number off of the starter. That was an easy find, but nothing else!

Any help on the location?

I've heard behind the starter, but I just spent an hour with a mirror, flashlight and screwdriver, getting all the crud off the block in that location. To my surprise, ZILCH! Nada! No comprendo!

Where the hell is this #??

Here is the serial # for my starter, don't know if it helps for decoding my serial # for my block haha.

D8AF-AA-8C2B