I'm running 17x8s with 235x40 on front and 245x40 on the back. When the fronts wear out I'll probably move the 245s to the front and go with a bigger tire in back. Maybe a 275x40 but that won't be for a year or so.
It is not so much the diameter, I've seen 5th gens with 19" rims on them, I didn't care for the way the truck looked with them on there but then that is a matter of taste. Billet Specialties recommends no more than 8" but you can go to 9.5s with the proper offsets. The risk, if it bothers you, would be the rims/tires will protrude the fender wells.
I have 16X8s, wanted 16 X 9.5s but Billet Specialties, like I said recommended not to go that wide so I didn't.
Check out these Jesse James wheels on a 5th gen. Elky. http://www.jessejameswheels.com/vehicles.htm
(the blue one at the bottom right). I believe these are the 18" ones. I personally wouldn't go this big on an El Camino.