[roger0080]

Oh ye children of little faith...

From the 1970 Truck Dealer Book, January 30, 1970 for C-K20 Options:
"Tank, Auxiliary Fuel: Available on C20 Fleetside Pickup models with side mounted wheel carrier only. Not available when paint stripes or custom upper side moldings are ordered. Includes 20.5 gallon capacity frame mounted tank located between side rails ahead of rear crossmember with floor mounted manual selector valve located next to driver's seat and fuel gauge selector switch located on instrument panel...........N18"

And yes, I do have a photograph of the N18 option on the spid.

If your spid is shot up, the best way to tell if if yours was a factory job or a hack job, remove the rubber neck tank seal from the fender and look at the quality of the hole in the fender. If the hole is smooth and balanced like it was stamped, then it's factory. If it is jagged, rough, rusty, dents, oblong, then it's a saber saw hack job conversion. Since you have a 3/4T then it's even more in your favor.

I've done this conversion and it's tough to get a nice professional hole cut into the fender with all it's curves. Plus it's covered up so it doesn't need to be perfect (especially on a daily driver as it was).