Since you don't already own the 6, I suggest waiting for an 8. It would be a lot easier to replace a small block v8 than the six. I bought a 67 with a 6 and 3 speed a few years ago, like you, I wanted to put in a 390 that I had in storage. After driving it for a few months, I decided to keep the six the way it is, and I might buy a V8 model if one comes up for sell, pull the 302 and drop in the bad boy.