/6 or v8 swap?
If you do go with the /6
@Rainy Day Auto hit it well in her post, I did the exhaust upgrade and the re-curved distributor I had a member do here, it woke it up a bunch, then I added a fresh air breather system that I feel the /6 loved as well.
A breather from a Jeep and an old carp hat to a 390 cfm, it was a much better driving car.
I would go 318 if it was me, I thing the high way and fun driving will be more enjoyed with the 318
Here is what I did, in pictures you will see a slant six manifold that was split with 305 chevy exhaust flanges welded to them, and ran 2" pipe to a single 2 1/2 pipe to the back bumper with a muffler.