It is in a different place on the early trucks, it is also a short 3/8x16 threaded bolt. On the later trucks it is a 3/8x16 stud that threads in to the head with a 9/16 hex and a metric 8x1.25 thread that takes a nut to fasten the strap to the haed.
Yes I just finished taking it off you're correct.
For our googlers looking for help:
On a 1990 350 the ground straps from the frame to engine head and the one from the firewall (under heater core hoses) to engine head are secured to a 14 mm bolt on passenger side head, between head and firewall. If you have big hands, you may have to remove a bracket (for plug wire holders?) also on the head and in the way, mounted with two additional 14mm bolts. Access to the ground strap bolt is precisely blocked by the transmission dipstick tube. At least that will help you located it, lol.
Btw, 14 mm is the size of the head of the bolt. It is not thread size/pitch.
Most efficient tool to remove would be a ratcheting swivel wrench.
You can do it with a standard wrench, but you'll be unhappy for a few minutes. =-)
Hope this helps someone.