Best year for a 318 build?

A running 68 could be a keeper. They were one of the highest compression teens built. With a head swap you can put together a 9.5/1 engine and make 300 horse pretty easy. You can do that if you are going to rebuild one as well with the right pistons.
a 84 to 91 came 9.2 to 1 factory...and 302 heads to build economical 300 hp mill as easy as falling off a turnup truck!