I just called Tanks, Inc and they don't have any plans for a tank for '78 to '87 El Caminos. But, they said they would put it on the list.
This is turning out to probably not be worth the effort for me to do right now. If I wait long enough, they will probably reproduce the FI sending unit. Or, some company might start making a custom tank. I could probably have Rock Valley custom make one but that would be very expensive and I can't justify that.
I guess I stick with an external pump for the time being. I do have a new Walbro external in a box that I bought a while back that I can put on it. I don't even know what brand the one I have on it now is. It is still working but I don't trust it.
I've got the car up on jackstands now doing a complete brake job. I thought it would be a good time to put in an in-tank pump. But, I guess not.