You nailed that. Run what is available, and 318s are definately available. A mild build with a bit of a cam, pocket porting and port match, 4V carb and headers will make a snappy street car.
Right. I encourage keeping the slant 6 too. If someone wants something cool and old to ride around in, go for it. If it's fixed up nice, more people will be around it at the cruise in! Ask me how I know! I have people tell me every time I drive Vixen to town they don't believe it's a slant 6 until I open the hood and they stand there dumbfounded and say "I never heard a slant 6 sound like THAT!"