To answer the OP's questions.....usually there's nothing special about a '68 318 engine. It could have a forged crank or cast crank. Some manual trans vehicles (mostly trucks) came with a forged crank. Most automatics came with the cast crank. All the forged cranks I've seen have a finished pilot bushing hole, Most (but not all) of the cast cranks that came with an automatic, didn't get that final machining for the bushing. Nothing special about the rods. You're probably going to bore it anyway, so, existing compression ratio and pistons don't matter. Put in what you want for the use you want to put it to. The stock 675 casting heads work just fine for most street applications. Full race, get some nicely ported aluminum ones. The 302's have some advantages in stock flow and mileage over the 675's, but, nothing remarkable. You can rework the 675's to improve both too. 302's come with hardened seats, but, there again, you can have those installed in the 675's. The smog ports would have to be plugged in the 302's if you're going to use the stock exhaust manifolds. I have seen lots more cracks in the later heads than the 675's. Not sure if it's the castings or the hotter conditions the later motors are subjected to. Also, here in Ca, we have a special blend oxygenated gasoline that runs hotter combustion temps (and worse mileage). Makes cracks in heads and manifolds much more likely.
