I am pretty sure that the Elkys had either an 18 or 22 Gallon cap. I think the V8s had the 22 gal and the 6s had the 18. Other than that you'd need to find the build sheet or run it out of gas then fill it up.
Lots of time GM left the build sheets in different places in the truck. Sometimes under the seat, some in the door panels, some under the headliner etc... not every car or truck has them for one reason or another.
The 4.6L V6 had 22 gallon tanks also. That was the tank that I bought for my El Camino so I had all of the baffles in the tank for my Fuel Injection setup. 17 and 22 were the sizes avaliable. I don't know how to identify each though.
The standard gasoline tank is 17.7 gallons. The 22 gallon gasoline tank was the N23 option. Unless of course if you have a LF9 5.7L diesel engine where the 22 gallon tank was standard. Look at the RPO tag in the glovebox to see if it mentions N23. If it's not mentioned then you have a 17.7 gallon tank.
It makes no difference if it is a V6 or a V8. The N23 option could be ordered with any gasoline engine.