This is what my '66 LWB fleetside looks like under neath.

http://img153.imageshack.us/my.php?image=my66rear01c1cn.jpg

and here's is the chassis notch

http://img76.imageshack.us/my.php?image=my66notch01c2th.jpg

before you ask, it was like it when I bought it, and I have no idea where you might buy the parts