No 318 ever came with 308 heads, unless it was a factory mistake. If you have heads off a late model 318, they are 302s, or there was another head with different casting number but same heart-shaped combustion chambers.

The 302s I'm running right now are from a 120,000 mile Fifth Avenue and they were not "rebuilt". Guides were still in tolerance and the seats looked good. I did cut down the valveguides so I could run a bigger cam later, and added new valve stem seals and Crane 308 springs which will be okay if I ever get the roller 318 cam reground. Then I did a whole bunch of home porting.
I installed them with Mr Gasket thin head gaskets. I'm still working out the carburetion, but on its first long trip it returned something over 21 mpg.
If you are absolutely bucks-down, find that late-model Fifth Avenue and pirate the heads, or the whole engine if you can. My shortblock had no ridge and the crosshatch still visible in the cylinders. Bearings were OK, too.

R.